Distributed Bass Array configuration

I have found the best and most efficient way to set up a sub is to measure first to get things in the ball park and then fine tune by ear from there.
What I get out of the whole DBA thing is measure before doing anything. It may be a waste if peaks are not a big issue. There will always be some though so it’s all a matter of judgement.
If the goal is to smooth things out at just one spot, DSP is probably the most straightforward solution. Or a good old fashioned analog parametric equalizer.
For smoothing out across the room then the more heavyweight dba solution might make sense, but only if the mains are capable of delivering across the room as well ie they have wide dispersion characteristics otherwise you might have smooth bass across the room but everything else falls short.

Then there is the alternative where you have multiple subs set up not  primarily to smooth out response but for best soundstage (smoother response is a secondary benefit).  That means if 1 sub front dead center,   2 subs at or near position of mains, 4 subs the two behind you are set to a lower level than front.  

I agree, level matching the subs to the mains is a good starting point, especially from someone installing their own subs. Where you end up doesn't mean the same position is necessary. At least you'll know where you are with reference to increasing or decreasing gain. Each time you add an additional sub, the SPL increases. So if having 4 subs (or more) in a room, I would think level matching becomes even more important, especially, as you said, if you have a difficult room.

Flat in room and or at listening position is just a starting point for great bass. Time, phase and Harmonics and the superb transition to mains with high pass is crucial. More sources ( subs ) in the room is a simple averaging machine for amplitude… great for sine waves… not so great for the rich harmonics above the magical oft cited 80 HZ line.  This is a big hint…. Play a decent recording of a scaling acoustic bass w eyes closed… image move ? (!Wales gets it, Eric surfed up next to it w Orchestral, a small Cello ensemble… I am guessing it aint all mono for them ) Glad the OP is at 95%, wonderful place to be but like any good audiophile questing for even better !!
of course use test tones, rta, good microphone… and listen. 
Fun thread but needs more cowbell and bass players…

audiokinesis

Regarding setting the levels ... finding the exact setting IS critical. The reason is, the ear’s heightened sensitivity to changes in SPL at low frequencies. A little bit of change in gain on the subwoofer amp translates to a disproportionate change in PERCEIVED loudness ...

That is completely false, notwithstanding your use of ALLCAPS for emphasis. See any basic audiology text for proof, which will show the range of increased sensitivity is typically in the midrange, 2000 - 5000 kHz. And that’s why the midrange of a hi-fi system is so critical to fidelity, even for those of us who also expect to hear the bass.

There is just so much misinformation about LF on these threads, but this was an especially glaring error.

@cleeds wrote: " That is completely false, notwithstanding your use of ALLCAPS for emphasis. See any basic audiology text for proof, which will show the range of increased sensitivity is typically in the midrange, 2000 - 5000 kHz. "

My bad for not explaining where I was coming from.

I do understand what you are saying, but what you are looking at is where the ears are the most sensitive; that is, where sounds of a given SPL are perceived to be the loudest. And indeed that peaks exactly where you say it does (with the peak centered around 3.5-4 kHz).

What I’m talking about is different, and is not nearly so obvious.  I’m talking about the ear’s sensitivity to CHANGES in SPL (and I would use italics instead of allcaps if that was available here). A change in SPL does not produce the exact same change in perceived loudness across the frequency spectrum; it produces more of a change at low frequencies. Go from a low volume level to a high one and the deep bass goes from being inaudible to being thunderous. The unit of measure of perceived loudness is the "phon", and by definition at 1 kHz the loudness in phons is the same as the SPL in decibels, so the two curves share that data point:

http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/eqloud.html

Now take a close look at how the equal-loudness curves bunch up below 100 Hz. A 5 dB change in SPL at 20 or 30 Hz results in a change in perceived LOUDNESS of about 10 phons, comparable to a 10 dB change in SPL at 1 kHz.

This is what I was talking about, and once you understand that, it will be clear why a small change in gain on the subwoofer amp typically results in a larger than expected change in perceived loudness. Or, to put it another way, why it takes a while to dial in the gain setting on a subwoofer amp.

Duke