Why the fascination with subwoofers?

@atmasphere 
Ralph: if I'm taking measurements and measure a deep valley within the subwoofer frequency, would that be pointing to a standing wave?

@mitch2 
While many records have very similar low bass recording on both channels, to be certain it works you should play a summed up mono signal to achieve this.

mitch2,

     It was mentioned earlier that humans are unable to perceive true stereo in the bass frequencies below about 80 Hz.  Even if you were an exception, there's the problem that there's virtually zero musical content recorded with discrete L/R bass signals.  I'm aware of none in cd or lp format.
     I run my 4 subs in mono mode because of this.  As I've stated before, however, I do perceive the bass as stereo in my system on well recorded cds and 24 bit/96 Khz FLAC files even though I believe the bass is summed into a mono signal.  By stereo bass, I mean I perceive the bass as originating from the proper position within the sound stage illusion.  For example, the kick drum dead center and the upright bass a few feet forward and a few feet left of the kick drum. 
     I can only explain this by assuming that the higher harmonics or overtones of the deep bass fundamental notes's frequencies extend beyond 80 Hz and are being reproduced by my main speakers, giving my brain the clues it requires to associate the higher directional harmonic frequencies with the much deeper and non-directional fundamental frequencies reproduced by my bass system and determine the specific locations of the bass instruments.
    In your situation, since there are no recordings with discrete L/R bass channels, locating a sub in the middle of your back wall and running them all in mono would give you smoother bass that you'll perceive as stereo bass.  Adding 2 more subs, running all 4 subs in mono and positioning them where they sound the best to you will provide the smoothest, most natural, most dynamic and effortless bass that you'll perceive as stereo bass.  I'm almost certain it'll provide the best bass response you've ever experienced and it will provide sota bass response throughout your entire room, not just at a single sweet spot. 
     I can share a best practices method to optimally locate each of your 4 subs if you're interested.

Tim

Look, small bookshelf speakers begin to work effectively from 120-150Hz. I mean really flat characteristic. There are different reasons for it. So, subwoofer mast work to 160-180Hz in this case. Just if you install it at center between the main speakers, bass and main signals can look "separate". And the negative effect is just stronger if the subwoofer stands somewhere in the corner. The best idea is to use main speakers working at least from 40-50Hz and subwoofer just to 60Hz. There is another way - you can have small main speakers and TWO subwoofers, one for each channel. They must be installed under the main speakers. It is possible to buy not expensive devices (about $200 each) produced by PolkAudio company.
Vlad

Do you have to play a mono (i.e., summed) signal from all the subs or would there be value to trying the front subs in stereo and the rear subs or one rear sub in mono to better deal with standing waves?

I'd put the side subs in mono. That's the plan in my system since my speakers already go solid to 20Hz.

Ralph: if I'm taking measurements and measure a deep valley within the subwoofer frequency, would that be pointing to a standing wave?

That would be my first suspicion!
 

For example, the kick drum dead center and the upright bass a few feet forward and a few feet left of the kick drum.
     I can only explain this by assuming that the higher harmonics or overtones of the deep bass fundamental notes's frequencies extend beyond 80 Hz and are being reproduced by my main speakers, giving my brain the clues it requires to associate the higher directional harmonic frequencies with the much deeper and non-directional fundamental frequencies reproduced by my bass system and determine the specific locations of the bass instruments.

Yup- that's exactly how it works.

Dear @lewinskih01  :  First than all you ask to a man that's a seller always. That person not even has in his system that four subs array but already making  advise about and telling stupid things like that : " that's exactly how it works ", and this with out first hand experiences in his room/system. I don't care that he is a tube manufacturer because this at the end means almost nothing on what we are talking here. Audio kinesis is a seller too.

Of the people that I know is not a seller and has first hand experiences with more than two subs is noble 100 and he has no interest to sell you or at any one nothing but only shares his first hand experiences.

Btw, that tube manufacturer a few months ago posted in a hot discusion with me in the analog forum that he did not needs subs in his system because he has a very good system response down to 20hz with passive speakers. Obviously he has no idea about and even today he does not knows what's talking on this specific low bass/subs management and he will know when he finally has first hand experinences at his place, not before.

Measurements always can help but if I was you or @mitch2  I must ask my self:

with the two subs I have and at my seat position have I good low bass perception: tigth, full, no resonances, no overhang and pristine definition on that range and great midrange/highrange performance?

if the answer is yes then we have to add nothing, two subs are more than enough and if yes then that means that the subs location is just rigth. Will improves in remarkable way adding two subs for a way better  quality performance at my seat position?, maybe you even can't say was a true up-grade. With low bass your ears will know all about if you know how low bass  performs in a live event seated at near field position. If we don't have this kind of experiences then we need ( a must to ) to live that experiences before any move /action regarding low bass/subs. It's up to each one of us.

If you need more than a one listening seat position the a third or four subs could be better but even with 2 subs maybe you can ( ? ) do better  moving a little your subs today locations, not drastically, only little movements with. Additional you always can use a parametric eq. for those subs and all these before adding two more subs.

At the end the " move/action " we make belongs to each one of us.

I like the Harman white papers because are an abstract work/research/modeling that try to sell nothing only sharing first rate information about.

Here two links that can help all of us:

https://forum.blu-ray.com/showthread.php?t=48286

https://www.bringtheruff.com/using-multiple-subwoofers-to-smooth-bass-response/


@noble100 , I already " finished " my tests on that low bass stereo/mono recorded or not and the response on what we are listening. I will post about in the other thread following our dialogue.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.