The thing to keep in mind is the wavelength of sound at various frequencies...
And how it relates to the size of your room...
For example, at 20 Hz, the wavelength is around 50 feet....
So, if you have two subs that are 25 feet apart, they are 180 degrees out of phase, and so will cancel out...
But, at 50 Hz, the wavelength is only about 20 feet....
So the distance at which "in phase" becomes "out of phase" is 10 feet at 50 Hz and 25 feet at 20 Hz......
Now, here's why this is a big deal, ESPECIALLY WITH MULTIPLE SUBS.
Let's say you have a small room, ten feet square, and you scatter a dozen subs around it.
Now you carefully adjust them so they are all "time aligned" AT YOUR PRIME LISTENING POSITION.
(Remember that you can only "time align" multiple sources relative to a single observation point.)
At 20 Hz, in that small room, they're all going to be more or less in phase everywhere in the room.
Probably close enough that you can simply assume that they're all in phase everywhere.
(Because no two of them can possibly be 25 feet apart - and many will be closer than that.)
However, at 50 Hz, that isn't true at all, because many of them may end up ten feet from another one.
Therefore, there will be lots of cancellations, and additions, between various pairs.
If you're lucky the net total may be more or less flat.
But, instead of being a big smooth flat, it's going to be.....
"a massive combinations of a bunch of 3D comb filter effects that hopefully add up to flat".
So, in a tiny room, or at very low frequencies, you have a "everything is more or less time aligned" situation.
But, in a very large room, or at higher frequencies, you end up with "let's hope all the complicated interactions end up smooth when we sum them".
This suggests some interesting things.
For example, even if you have multiple subs, if you have a big room, you should consider putting them in the same area.
(That way they can be more or less time aligned with each other.)
And, if you have a big room, and you put a bunch of subs around the edge, you could STILL end up with a node or null in the middle.
After all, they're all the same distance from the center of the room, and the same distance from a wall... so not really "scattered".
(After all, most of us wouldn't even consider actually scattering a dozen subs EVENLY throughout the room... right?)
And let's not forget that ALL of those subs must interact with our regular speakers around the crossover point.
Of course, you could consider
NOT having all those subs, or groups of subs, cover the same range of frequencies...
