Why the fascination with subwoofers?

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?

@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.