This is the crux of the matter. One would want point sources, where possible. And outdoors, it's almost trivially easy to get point source radiation from woofers. Only when there is a large array of woofers do you have a problem, but a single woofer acts as a point source.
Indoors, that's not the case. Even a single woofer acts like multiple sources because of the reflections from boundaries. At low frequencies, multiple subs are used to provide dense interference. Dense interference isn't as good as coherence, but it is better than course interference. At least with dense interference, you can smooth the sound field, make it act a little more like the reverberent field.
At low frequencies, it is again almost trivially easy to get a smooth sound field indoors because you just put a bunch of subs in the room. They aren't localizable, so you can pretty much use a "more is merrier" approach. If you have four subs or more, it almost doesn't even matter where you put them as long as they aren't clustered together.
Where it's hard is in the midbass and lower midrange. Sound is starting to localize in this range, yet it is still modal. So the traditional multisub configuration is best modified with a slightly different approach, which is to flank the mains with corresponding helper woofers. As I said in my last post, what you want is to have them just a few feet away, a little bit below, behind and beside the mains. They should be blended up to about 150Hz, to smooth the self-interference from the wall behind the mains and the higher frequency modes, just below the Schroeder frequency.
This 100Hz-200Hz transition region is what flanking subs are used for. It's low enough that you are still dealing with discrete modes, but high enough that sound begins to become localizable. Even if there were just one woofer, it is still effectively acting like "multiple midbass modules" because there are multiple midbass modes, due to the reflections from the boundaries. So just like the multisubs are there to provide dense interference at low frequencies, we use flanking subs to mitigate problems from boundary reflections in the midbass and lower midrange frequencies.
Edited by Wayne Parham - 10/21/12 at 11:45am