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I'll put on my thinking cap and offer a long-winded reply to the reply

Bass varies in every room -- I can only offer theory (and experience for typical rectangular "plasterboard on 2x4" rooms):

YOU WROTE:
If I understand you correctly, you are saying that standing waves are the dominant problem in the deep bass region, and that subwoofers need to be located near the main speakers to integrate properly.

RG:
Correct summary of bass theory ... but never forget that measurements of the room and listening rule, not theories.

The best sub location for the bass frequency response may not be the best location for excellent integration with the main speakers. My first priority is near-perfect integration with the main speakers. That forces sub locations to "close to the main speakers" (two subs) or between the main speakers "one sub".

There are tricks to allow locating one mono sub near either speaker rather than between them (sharply cutting output over 80Hz.).

And if the sub has very little output over 40Hz. it can theoretically be located anywhere as no experiment I've ever read suggests localization is possible with sound under 40Hz.
... although with loud music content under 40Hz. it is possible to "hear" a sub is located behind you, or to your side, solely from the feel of bass pressurization on the side or back of your body.
That may be okay with surround sound but with two channel I don't want to hear the illusion that a bass player is stnding in front of me and that's not helped by feeling pressurization from the lowest notes on my back from a rear subwoofer.

I know I'm extra picky about hearing at home the bassline that the bass musician intended. The bass player almost always plugs his bass into the recording console because even a recording studio is too small a room for a good bass frequency response.
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If standing waves are the dominant problem, then changing the subwoofer's location shouldn't make such big difference because the standing wave patterns are a fxed function of room dimensions.
RG
Sub location determines how strongly a standing wave is excited, and listener location determines how well his ears will couple with the standing wave. Subs and listeners located far from walls/corners of rooms have a much different bass frequency response than subs and listeners located near walls/corners.
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But as we all know, changing the location of a subwoofer has clearly audible and even dramatic effect. The reason it does is the path-length-induced peak and dip pattern that I've described (and which Roy Allison described long before me, but he focused on the reflections off the wall closest to the speaker whereas I'm assuming the subwoofer starts out up against that wall).
RG
Not true.
Subs tend to be located near walls, or no more than a few feet from walls. This is too close to affect 14 feet and longer wavelengths?
Well ... it wouldn't be a good idea to locate two subs so their drivers were 4 feet from the side walls, 4 feet from the front walls, and 8 feet apart. That could cause a deep narrow null.
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I believe one key to good subwoofer integration with the main speakers is generating a low frequency sound field that is similar to that generated by the main speakers at higher frequencies.
RG
Impossible
Bass is omnidirectional.
Sound gets more directional as the frequencies rise.
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Scattered multiple subs addresses this. Given that the ear is very poor at judging the direction of a low frequency sound source without upper freqency cues (hence the steep crossover), and that the ear is obviously pretty good at hearing large peaks and dips in bass energy, I place the higher priority on getting the soundfield right.
RG
I wish scattered multiple subwoofers smoothed the bass response.
The soundfield is never right if there are any bass peaks +3 to +6dB from standing waves (because +6dB can means a bass note fundamental tone sounds twice as loud as the bass musician intended!) Add in a null or two and you hear a different bassline than the musician intended in our small rooms (compared to listening to headphones or listening to speakers or a live bass musician in a nightclunb or auditorium.) Very large home listening rooms tend to have reasonably accurate basslines unless the ceiling is under 10 feet tall.

Listen to s slow sinewave frequency sweep tone (at least 20 seconds from 20 to 100Hz.) and you'll hear whether the bassline is reasonably flat. It usually is not without many bass traps and/or parametric EQ.

The oddest thing about people experimenting with multiple subwoofers is they tend to place all the subwoofers on the floor where they fully excite the very important floor-to-ceiling standing wave (70 Hz. in 8 foot tall rooms, and I'm assuming the sub has full output at 70Hz). There are theoretical advantages with four subwoofers (left floor, left ceiling, right floor, right ceiling) if the ceiling doesn't rattle from the bass (it will) and the wife doesn't send you to the funny farm (she will) and you can afford FOUR subwoofers.
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Duke


Richard BassNut Greene
Subjective Audiophile 2007

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