What process did you use to integrate multiple subwoofers for 2 channel listening?

Today I will be trying to integrate up to three subs. Two are matching Rythmiks F12SE, and one is a REL R-328. The Rythmiks have a variety of adjustable parameters, including phase, crossover, and gain. There are other switches and passes on the sub, but I'm going to try to keep it basic to begin with. The REL has variable gain and crossover; the phase on REL is either 0 or 180.

I have REW for measurement. I will be buying a few more furniture sliders this morning, on doctors orders. ;-)

QUESTION: If you have multiple subs, by what process did you integrate your subs? One at a time? More? Which adjustments did you try first and in what kinds of increment?

I know that trial, error, measuring, and listening will all take time. Rather than look for a needle in a haystack, I'm curious what sequence or process was most effective for you.

Thank you.
My conclusion based on the physics, principles, and actual personal experience is this whole "integration" thing is a distraction, a time waster, and interferes in people's understanding of what is really going on. 

After years and years of trying one sub, as well as searching around reading about and whenever possible actually hearing all kinds of solutions, I had totally given up. Good bass cannot be done. It is just too big a nut to crack. 

When I heard about DBA around 2 years ago I thought here we go again. This time however it turned out there was a doctoral thesis written on this very subject. Not some phony baloney doctoral thesis either like the one recently which is beyond a joke. No, this was a serious paper by a grad student who actually exhaustively measured bass levels in different rooms with all kinds of subs and placements.   

What he found was remarkable: the more subs the more modes and the smoother the response. He even worked out a formula showing the improvement each additional sub contributes to smoothing bass response.  

Then to top it all off everyone who actually tried this method raves about it.   

Along the way I also learned some very crucial psycho-acoustics. To wit, humans cannot localize very low frequencies. Not only that, it takes us a lot longer for those low frequencies to even register as being heard. Tests with headphones prove we cannot hear 20Hz at less than one full wave.   

One full wave at 20Hz is 1/20th of a second. Duh. But now I want you to really think about that. 1/20th of a second. Sound travels roughly 1ft per millisecond. These are all just facts. Highly encourage everyone don't take my word for it, look it up. 1ft per ms varies by atmospheric pressure but who cares, it is a nice round number and close enough for government work.  

1/20th of a second is 50 milliseconds which equates to 50 feet. You can also plug it into this calculator, which will tell you it is 56ft. http://www.mcsquared.com/wavelength.htm  Close enough. 

So then, how on Earth is it possible to "time align" or "integrate" or whatever you want to call it something that we not only cannot locate, but cannot even hear until it has traveled clear across the room and back? Riddle me that one, Batman! We can't. I really do hope this demolishes the "integrate" and "timing" hypotheses.   

Not holding my breath. 

Also not being one to stay stuck in mental ruts I pulled myself out of that one and built four subs. When I got them in the room they were plopped down near walls and corners pretty much wherever there was convenient space. I did try and have them be different distances from corners. But that was about it.  

When I turned them on straight away right off the bat immediately and without doing anything I heard the best bass I ever heard in my life, and by a long shot. Not even close.   

THAT is how I "integrated": with physics, science, and logic. Plop em down. That simple. 

When you have four or more. The fewer you have, the harder it gets. NOT because of anything to do with integration! Purely to do with the physical fact that fewer subs results in greater lumps, bigger peaks and dips, while more subs results in smaller lumps, smaller peaks and dips. With four subs you get incredibly full yet tight, deep, extended bass, effortlessly. Both effortless in how it sounds, as well as being effortless in what you have to do to get it: plop em down. Yes it really is that easy.   

Now, once you do have four subs in the room sure, you can work on getting their level dialed in. You can kill yourself trying all the different methods. Or you can set the crossover to about 80 and then adjust the volume levels. If you have something like the Dayton SA1000 then you can also use the one band EQ, bass boost, and or low cut filters to tweak things.  

I highly recommend doing all this by ear. Because, see Equal Loudness Contours. Meters measure equally regardless of volume. But our hearing changes dramatically with volume. So you can have it measure flat, but it will only sound flat at one volume level. Above that it will sound like too much bass, below that too little. So it is a trade off compromise situation. 

In my case, I made some pretty big swings early on, then within a few days got to where I was making smaller and smaller changes further and further apart.  

You can get more detailed about it but this really is all there is to it. Listen to a lot of different music over a long enough period of time and then make only a very tiny change. Because you will find some records will sound like there is no bass, the subs are doing nothing. This is the way it should always be. You do not ever want to feel like you are listening to subs. I sure don't. Read the comments. Most people don't even know there are 5 subs in the room until I point them out.

Tim, Noble100 used the crawl method. I tried that, you can do it, but read up and notice we all wind up with pretty much the same thing in the end. Then we all wind up making tiny incremental tweaks until finally at some point, done. 

Sarjan at 6moons has several articles and reviews on integrating subs that are very informative, cutting through all the bunk. John Darko has done similar work as well and he even has videos about his quest to properly integrate his sub.

Worth checking out.

All the best,
I used REW for 4 subs. First I setup the front 2 by themselves and then the back two separate to make sure volume, frequency response was similar and then all four together. 

Thanks. I have 3 subs. @millercarbon, I'm not sure what mistake you're trying to disabuse me of, but all I mean by integrate is make them sound good. Some of the technique won't just be listening but much will. From listening to moving/changing to measuring back to listening. 

you can set the crossover to about 80 and then adjust the volume levels.
That's a helpful bit.

@nonoise  Thanks for the reference.
@mmcely Thanks for sharing your technique concisely. I'll try it.

Just after I posted this, I found this very helpful and detailed discussion: https://www.thehifipodcast.net/episodes/episode-24-how-to-integrate-subwoofer

I set up mine the old fashioned way - I put sub #1 in my listening position and walked around close to the walls while playing a track with deep bass.Quickly found the two corners where the bass was best and set them up there. Then it was a matter of trial and error over the next few days adjusting the settings.
I eventually moved one to a "null" corner firing towards the peak corner where it previously resided.I think it's smoother that way in my room. They are both behind me in the back corners totally unobtrusive. Anyone listening to the system marvels at the bass the little Harbeths kick out (heh,heh).
The problem I’m not able to solve, and perhaps the room’s physics make it impossible, is a +5db or more bass hump from about 73-92 hz. This is evident in REW and it’s also easily noticed with music. A tubby zone. The idea that one more sub would solve this is implausible. It will take a systematic approach to moving the subs and adjusting various things, and I'd bet a few hundred $$s that that fails too. But at least I'd know I had hit the limits of this room. Thanks for any approaches. I’ll ask over at the REW forum, too.
I think you've nailed it your thread description - trial and error. It takes a lot of time to go through the various iterations, but viewing the measurements is well worth the effort.
If you haven't done it already, go to AVS forums and to the Rythmik thread and download Enrico's tip for dialing in their subs (it's in his signature).
Don't be afraid to use different volume adjustments, different frequencies, different bass extension filters for each sub. Experiment with room locations (ie different angles, even pointing the front of the sub to the wall).
Take notes (I added them to REW). It's all important so you know what works and what doesn't. Taking notes will help speed things up. Carpet slides are a great idea.
Are you running any software you can apply EQ, should be fairly easy to notch
 70-90hz.  If you have the PEQ version of the Rythmiks put a -6db at 80hz,  measure and adjust. 
The problem I’m not able to solve, and perhaps the room’s physics make it impossible, is a +5db or more bass hump from about 73-92 hz.

Well, this is easily solved by an EQ, and that is a relatively small hump, I've measured +20 in a modest living room at ~ 25 and 50 Hz. 

This hump is also in the range of bass traps such as GIK's.  Clipping it will enable you to bring the bass level up and sound more full and balanced.

Rather than try to solve this with multiple subs I would strongly encourage a mix of bass traps and EQ.
I’m using a pair of 15" Rythmiks to supplement bass on my Avantgarde Duos, which have integrated woofers that do not go down extremely deep- when they are dialed up for more bass impact, they sound discontinuous with the mid-horn; when adjusted to blend with the midhorn, they sound anemic. The Rhythmic subs address that pretty effectively.
My task was made easier by focusing on the blend between the integrated woofers and the subs-- partly adjusted by positioning of the woofers (I had help in the form of two college football player types who did the heavy lifting). I also used a cheap little DSP unit- the Dspeaker 8033-- which is surprisingly effective to even out the bass in a decent sized room.
Once I ran the cycles on the DSP unit and it lopped off some peaks, I then zero’d in more-- lowered crossover frequency, adjusted slope and played with phase to get them to cohere. I had help here-- my wife- who adjusted gain, crossover and phase with direction from me sitting in normal listening position.
The system is very resolving at low volume and when the subs energize the room, you know it! But, you almost would not know they were there unless I shut them off. (FWIW, they run from a separate line out on my line stage, so the DSP is not in the same circuit as the main speakers). Even if you do measurements, final tuning is probably going to be by ear, no? Once set, I’m done. I do not mess with the settings for different kinds of music. The DSP unit sums the channels so it's mono. I know there's an argument there, but it is way easier to do and my main source is LP, which is often summed below a certain frequency anyway. 
Love to help you hilde45 but helps a lot if you a) ask what you really want, b) use words to mean what they actually mean and c) pay attention to the answers. 

We are now down to a very specific problem, "a +5db or more bass hump from about 73-92 hz" at least until it changes again. Main obstacle now being c) pay attention to the answer. Will you do this? Because I am not about to waste my time on another one where someone has already made up their mind. Why? Would you? 
Search audioKinesis posts and you will likely find the info you need.

The key I seem to recall is to have the subs in the right locations to randomize the bass waves to provide a smoother response throughout the room. Might be harder with 3 versus 4.  
MiniDSP makes a unit that integrates up to 4 subs.  Look on YouTube for the lengthy but detailed instructions.  Mostly for Home theater but concept is the same. 

Pardon my probably obvious questions. I’ve been contemplating how serious I want to get about subwoofers. How many and what size and what budget and where I’ll put them and how I’ll connect them etc, etc.

is the 5dB bump there even without the subs active? (Is it the room or the subs?) Can you drop the crossover frequency enough to effect the bump?
Trial and error!  It helped me to just have the sub playing (no mains) while moving it around the room to find the smoothest, most even response.  I have a bad null in my room that I had almost given up on until I put the sub directly behind me.  
Subs for me are a love hate relationship.  If not set up properly the can actually make a system sound worse.  Properly setup though, they can be a game changer. 
Subs are like wife's..can't live with them or without them..the trick is to spend some time with them. Figure out what works and what doesn't...relative to what you want out of the relationship...big bass big impact..multiple subs with little but subtle immediate impact...one sub...move it around to see how best it fits relative to your environment....? Are we talking Audio technology or being married..or maybe the relationship is the same?  Been in this hobby for a long time and I think there is a lot of similarities in wanting that extra something (i.e. bass) to make our system more than what we expect. Just saying 😌
I have learned to respect millercarbon's opinions. Also, I have traveled down the road to the integration of subs.

If you have the patience to really understand the science about sub placement, settings, etc. read this article. It's quite large but explains a lot about using subs.......

@golfnutz Great tip. I’ll head over to AVS and look for Enrico’s tips. I’ve been playing with a lot of parameters but not the filters as much. I thought those might be later factors — too complicating, early on.

@djones I’m not running any software that applies EQ. I was hoping to avoid adding that to the signal chain.

@whart — it may be that DSP is a necessary step. Thanks for your detailed process. There’s a lot for me to try in there. And yes, listening will be the final test, and at many points along the way.

@fittebd — thanks. Will check it out.

@jrpned Thanks for the white paper.

@miller the hump is one main problem. You are saying that (a) I don’t know my own goals or mind, (b) don’t know how to use words and (c) don’t pay attention. I think the final answer is actually not listed — that you don’t really want to help. You want to find clever ways to poke at me. I’d prefer for you to simply stop posting on my threads. The whole tone of conversations without you are more amiable and relaxing.  Absence makes the heart grow fonder. I think I'd like you more if you were not around. 

@mapmapn, I’ll take a look at audiokinesis. Thank you. I realize that putting them in various places may be key. I worked all day at that — and didn’t get rid of several troublesome areas.

@erik_squires I have a lot of GIK stuff, as well as Real Traps. I found I had deadened the room too much in the 1-4k range and so had backed off. Some of the traps have limp membranes so perhaps some rearranging could help so I can do as you suggest without killing the "air" of the room.

Im running 4 rel subs in a room. They are all crossed over around 55. Very low gain. 
Its awesome. 
@hilde45 -  The problem I’m not able to solve, and perhaps the room’s physics make it impossible, is a +5db or more bass hump from about 73-92 hz. This is evident in REW and it’s also easily noticed with music. A tubby zone. The idea that one more sub would solve this is implausible.

I have the Swarm DBA from AudioKinesis in my system.  There was an issue with shipping causing the Dayton amp and two subwoofers to arrive a few weeks before the rest of the system.

Not wanting to wait for the other two subs, I hooked up the Dayton and one sub at first.

Yes there was an increase in bass quantity but not quality.  The next day, I connected the second sub and noticed an improvement.  Bass was less bloated and seemed better timed.

When the other two subs arrived, I connected a third and again noticed improvements.  Perhaps the same type of improvement to a lesser degree.

After connecting the fourth sub, I couldn't hear anything negative with the bass response.  No bloated or slow bass.  Even the higher frequencies showed improvement.

I have used the DBA in a small 11 x 13 (minus closets) room and also a 12 x 15 living room at my old place with outstanding results.

The DBA continues to shine in the 15 x 19 listening room I have at my new place.

Hope this is helpful to you!

Get in touch with REL. They have technical specialists that will help with optimizing your setup
I have over the past year added 3 subs to my system 1 at a time. Placement was not as much an issue as volume level and low pass set point. For me both went down quite significantly over time. You might save yourself some time and start out with a lower gain and low pass. Good luck.
It's a mistake not to use digital room correction in addition to several subs and sub placement. If you add it to your digital chain it will only improve your sound.

For me Dirac bass management plus additional (to my hearing) Roon PEQ filters works fine to give impressive lower and upper bass.

If your only concern is a +5 db hump then you're really fortunate. More normal would be +/- 20 db between 20-100 hz. With one or two nulls.
@hilde45 You don't need an EQ for your +5db hump (and that is actually a work around not the actual way to deal with it)

The hump occurs because the frequencies are being summed from your subs and your speakers (which will be different for each sub/speaker/room combo)

Your Low Pass for the sub needs to be lower as I am assuming that you are not going to rewire your crossovers in the speakers. If you have it at 80 currently (as another person suggested) I would say lower it to 55 and remeasure to see what happens. If it turns into a slight valley turn it up to 60. 
@hleeid I'm convinced I'll do a DBA, I just can't do it yet. I'm in a temporary room and will probably be able to move to another but not for 5 years. This makes a DBA impossible. I'm doing my best to optimize with 3 subs. If that is still 1 card short of a full deck (i.e., crazy) then I'll realize that and in 5 years will be able to correct the issue by selling the subs used and going for an array. Given the wide range of controls on these Rythmiks, I'm hoping I can do pretty well until I get an array.

@jmalen123 and @powemi2 Thanks for the suggestion process for the low pass.
@mapman, thanks for the Ohm stuff. I'll take a look.

FWIW, here's what I was able to accomplish after 6 hours yesterday:

5db null at 33 hz
6db peak at 77 hz
5db null at 155hz
5db null at 174hz
9db null at 208hz
10db peak at running from 379-594 hz

I put the graph on my system page: https://systems.audiogon.com/systems/9064/edit

hilde45, if you really want to do this right you buy a digital preamp with digital bass management capability like the DEQX Premate or Anthem STR. You can set your crossover point and slopes independently then set delays so the subs match your main speakers in time and phase. Now you need only passive subs which opens up a whole range of DIY capability. You can actually build a better sub than a commercial unit because problems like shipping weight are no longer an issue. There are great sub drivers available. Otherwise you just bypass all the cheap front end garbage in the commercial subs and use just the amplifier section. But, outboard amps are much better. If electronics are so susceptible to vibration is putting the amp in a vibration machine a good idea? 
For starters, I would only use the 2 Rythmik subs to begin with. I think once you have these dialed in, you can add the REL sub to see what affect it has, and start tweaking from there.

If you download Enrico’s dialing-in document, you’ll find you need to set the Phase correctly, and Volume match your subs to your mains (pink noise and SPL meter required). Without doing these, you’re wasting your time with guessing. If you do this, you might find it easier to fix that early dip (use the extended bass filter to see how you can bump it up). Don’t adjust both subs the same all the time, experiment with one sub only, then set it back and try the other sub.

You should also load a graph of your main speakers only. I think you’ll find the same results as the graph you posted for anything starting over 150-200Hz (I’m not sure what the cut-off is on your subs). Basically, the subs aren’t going to fix this.

This is without any DSP, just location and adjustments on Subwoofer - https://imgur.com/a/G9kHdTt

I had to back off the low end frequency (20Hz - 30Hz) because my wife was complaining it was making the house shake too much.

If it were me going through the time and expense of trying to setup multiple subs I would likely use a minidsp device to do it properly. 
For everyone saying 

“to do this right you need X” 

He has a mic and software with subs that have a low pass control. That’s all you need. Nothing else fancy especially a dsp with converts analog to digital to analog again. 
How much did you spend on your DAC and you’re trusting the DAC inside the DSP to reinterpret it all?  
All those extra things are for people that want the easy fix without actually gaining knowledge on how your system works together.

Again doesn’t need anything new (I understand people love to throw money at a problem but it could be fixed with what he has before the shipment arrives); just spend a little time with the low pass filter while looking at you readings from the mic.
Minus 5 db is not a null. It's a small dip. A null is an infinite dip. Your problems seems to be more in the high bass/low mid. If you can trust your measurements.....
Thanks to all for your useful posts. I appreciate them.

@golfnutz Good point — it seems like you’re suggesting start with one — using Enrico — then go to two, then three. I have the tools needed to do what you suggest. Comparing to the naked speakers is a good idea. This is what I’ve done in the past. (I have been doing REW sweeps since July of last year, including with moving the sub. At one point, I had the sub 4 feet off the ground to check for vertical effects.) I like your SPL graph. As for shaking the house, isn’t that our duty as manly-men-with-subs? ;-)

@mijostyn  I do really want to do this right. Because I just bought all my tube gear and powered subs, I will probably wait before buying a digital preamp with digital bass management capability and then selling off my active subs for passive subs. I see that if you’re right about this, I’m going about it the wrong way. But for right now, I’m looking for solutions to what I currently have. Down the road, perhaps I’ll change everything out for something you’re describing.

@mapman Mini DSP would be a decent experiment. I just have to absorb the idea of adding it to the signal path. It’s a question of which is worse — adding another thing or some bad curve sections.

@powemi I have a DAC I really like (MHDT Orchid) with a good tube and really nice upgrades by Grannyring. I really incline, as you say, toward trying to learn my room and my system. I want things to fit together organically, and I honestly enjoy moving a panel or twisting a knob and noting the change. I get in a headzone of iterative trials — I can go for hours, and all the while, I’m learning how physical space, quantification, and acoustic experience align. I would worry that a DSP would shortcut me out of this process. Still, I’m losing hair and if I pull too much more out…well…

@tvad Thanks tvad. I don’t know why Duke would want to advise me when I’m not buying anything. He’s got to be busy enough.

Post removed 
I am not going to say I am an expert in any way. Some of this can get expensive because subwoofers are not cheap. I have found that trying to control sound at the lower levels can be the toughest thing to control. I have also found out over the years there are equipment audiophiles (likes to tinker with equipment to change the sound), music audiophiles (only wants to listen to good music), and a combo of both. I am both. Take on one sub at a time with the sub crawl. Best of all it’s free except for your time. Read up on the papers and forums to try their ideas. Again it should be free. Finally buy a Mini DSP or crossover to help you remove the humps or nodes. Just remember our biggest issue is the room. You might want to invest in some bass traps and / or diffusers. The node or hump you are talking about is not the frequency that bass traps fix. They are usually above that. I’m speaking in general. This can be frustrating especially if you are not an equipment audiophile. Finally you can hire a guy if you have the means. Usually it’s $100 per hour in the Chicagoland area. MC may not have patience but he did tell you what to do which you gave someone else credit for the DBA. Basically the more subs you have the less volume they play so those waves don’t have to travel as far which reduces humps and nodes. Try the first one next to you listening position pointing forward toward the front speakers. Then one in opposite corners of the room back to front. Think about it like dropping a stone in a pond at one end. The waves remain big and travel all the way across the pond and back again.  Now two people drop stones at opposite ends. The waves get smaller when the meet. As you drop more stones the waves get smaller or canceled out. I hope this helped simplify the understanding of controlling bass. 

Just a friendly reminder. 
Use your ears. 
Just because something measures well doesn’t mean you’ll like it. This is the reason I say it is best to use your mic and play around with Low Pass to see what it does. You may find that you want a +3 db in that range -for your ears- or you may find you prefer a -4 db. 
It is your system so use your ears; what sounds good to you may not to someone else and I feel we lose that thought process with all the tech talk in the community. 

Saying that something measures well is different than saying it sounds good. 
Your ears are different than mine than his than hers. We all can’t use the same pair of glasses either. 
MC may not have patience but he did tell you what to do which you gave someone else credit for the DBA.

Proper credit where credit is due.

MC learned to build his DBA (distributed bass array) from Duke LeJeune. Check MC's posts.

Duke has been designing, selling and discussing distributed bass arrays since at least 2014. He credits Earl Geddes, and states he is using Mr. Geddes ideas with his permission. Search posts from @audiokinesis.

Like in this thread:

04-06-2014 2:05am

Ime when it comes to low bass, the elephant in the room is the room. No matter how flat the subwoofer starts out, the room will superimpose its effects. We can change the room-imposed peak-and-dip pattern by moving the sub and/or moving our listening position, but we cannot make it go away. It is impossible to find a location for a single sub that provides approximately flat response in the sweet spot, much less elsewhere in the room.

Distributed multiple subs offers an elegant solution, each sub being in a different location and therefore generating a different peak-and-dip pattern, but the sum of several such dissimilar patterns will be vastly smoother than any one alone. Instead of a few big peaks and dips that are quite audible, we end up with more, smaller, closer-spaced peaks and dips, and the subjective improvement is even greater than we might expect because (at low frequencies) the ear/brain system will average out peaks and dips that are within about 1/3 octave of each other.

While it’s possible to equalize a single sub to be flat at the microphone location, its response will be far from flat at other locations within the room. A distributed multisub system significantly reduces the spatial variation in response, such that the difference from one location to another is greatly reduced. If further EQ is needed, it is likely addressing gentle global problems, rather than acute local ones.

As a general rule of thumb, two subs have about half as much variation in in-room response as a single sub, and four subs have about half as much in-room response variation as two subs (assuming they’re spread around somewhat). Smooth bass = fast bass, because it is the excess energy in peaks that makes a subwoofer sound slow (the ear/brain system having poor time-domain resolution at low frequencies, it is the frequency response that dominates our perception). Also, smooth bass = powerful bass, because we’re likely to set the average level of the subwoofer lower than it should be if the response has distracting peaks that stick out like sore thumbs.

Dipoles have smoother in-room upper bass than monopoles do, so the discrepancy between two dipole mains and a single monopole sub is greater than what we get with conventional main speakers. If you read accounts of people who have tried subs with dipole speakers, it seems like most people who try a single sub eventually give up because they can hear the discrepancy. Most people who try two subs keep them, because they don’t hear much discrepancy. Three or four subs would be better still, and they can be small subs.

I spent several years trying to design a super subwoofer that was "fast enough" to keep up with Quads and Maggies while offering good extension and impact. I tried sealed, transmission line, dipole, isobaric, aperiodic, and more. Then a conversation with Earl Geddes changed everything, and I’m now an advocate of distributed multiple subs, in particular for use with dipole mains. I’m using his ideas with his permission.

Imo, ime, ymmv, etc.


You can of course do it with a test signal like white noise and a meter. That’s how I did it with 1 sub. Learned you can’t just trust your ears in this case to get it right. More subs and more controls means more complexity. That’s when automation like minidsp comes in handy to cut to the chase. Live and learn …. that’s a good thing.
I thought the Ohm blog article I linked to by John Strohbeen above was interesting. Something of a different take on optimizing sub placement for audio as opposed to home theater with more of a focus on timing of the sound waves from the subs relative to mains and listening position and how that affects what you hear than what I’ve heard from bass array proponents elsewhere where the main goal is to produce smoother bass response throughout the room including main listening position. JS’s conclusions on placement based on that seem simpler and different from other bass array proponents including Duke at @audiokinesis. 1 sub, center forward. 2 subs as stands for mains. 3 or more to the rear and set to lower SPL. Would love to hear AK’s take on that?  It seems more focused on optimizing the bass more thoroughly specifically at the listeners location.  

One other thing. I know audiokinesis is big on speaker dispersion patterns and how that affects the sound.
Seems to me that benefits of a bass array is not the same in all cases. Mains need to have good dispersion characteristics to start with to match the subs I would think for best results. If mains are highly directional, there may be only so much to gain from smoother bass response across a room. Or it might just make getting things to sound right across the room impossible to start with no matter what you do with subs.
Food for thought.

@tvad Thanks. I didn't know I could do that. Makes sense. I was under the impression that MC invented the DBA and Duke just happened to be selling it. His system page puzzled me because it seemed like I should be seeing Duke's stuff in his room but then I realized he made it himself.

@hshifi Thanks for your input. I'll try the crawl again but with three subs, I better get my knee pads. Another vote for DSP -- thanks! As for bass traps or diffusers, I have many -- 4 Mondo Bass Traps, 2 GIK corner traps, 2 GIK Q7 diffusers, 4 GIK HF limited bass traps, 2 DIY absorbers 4" thick, 2 ATS bass traps, 4 ATS absorbers. I've also made several more pillows (e.g. a giant dog pillow stuffed with R38). I've moved everything around everywhere and measured all the while. Dozens of hours or more. I've moved huge amounts of furniture and even brought in more bookshelves for more diffusion. Plus, so homemade deflectors home made from wood boards to tweak reflections.
Hiring someone may be the way to go. I understand how multiple subs are supposed to work and how bass waves move in the room. It's now a question of discovering what's efficacious, given all the factors.

@powemi Good point. I will use my ears. I'm developing a sympathy between measurement and listening. The reason I am mentioning the bass hump is that I can defnitely hear it. The problem with using one's ears for a deep null is that one may not realize very easily that there is missing information, because we tend to fill in the blanks with our brains.

@mapman -- exactly. It's the complexity that makes a sub crawl or just using my ears hard to fathom.
SO the one thing that should be gained rather simply with multiple subs no matter what is that each sub now has to work less hard to provide the proper bass level and as Duke alluded to in that original post as provided by Tvad, that alone should be a positive to some extent in that most amps sound better when they do not have to work as hard. How much better? I suspect YMMV there case by case.

So more subs are better than less, but you do have to get them all working together properly somehow. You can’t just assume well its bass so location does not matter. Nothing truly worthwhile is ever simple, right?

Also it will most likely never ever be perfect still. Just better. Something no self respecting audiophile is ever likely to find easy to accept. It can always be even better, right?

Where does it stop? You get to decide. I’ve stopped at just one when needed (only with limited range speakers) and 0 with full range. The most I see myself ever going if I convince myself I need more torture and clutter someday would be 4 based on what I’ve read to-date.
Yup, I knew I would end up with a pair of subs.  I am a big fan of the Vandersteen subs.  I think their unique design makes for excellent integration and easy placement.  They are designed for corner placement (why isn't every sub designed to sit in a corner?).  I had to buy used, so there was time between the first sub and the second.  The first went in one corner behind and outside the right speaker, the second went in the other corner behind the left.  A little tweaking of the level and Q controls (crossover is fixed at 80Hz, first order), and I was enjoying deep, tight stereo bass that sounded like it all came from my mains; these subs never call attention to themselves.  Fantastic!  I have the discontinued 2Wqs, but the newer model has a built in EQ, which should make it even easier.  
Rat-shack sound pressure meter, test LP and lots of experiments with positioning of the two dual 18” DJ subwoofers