xcool - Our ears aren't good at locating low frequencies so subwoofers don't contribute to the stereo image. In fact you will probably find that the best locations are not near your speakers or symmetrical.
What you're looking for is for each sub to excite different room modes so that they balance one another out to some degree. If you're not familiar it might be worth looking up 'room modes' and 'standing waves'.
Excellent answer. My system is a great example. There are 5 subs asymmetrically placed around the room with each one a slightly different distance from a corner than the others. With music playing, even with plenty of bass, you can walk right up to any of them and think it is disconnected. In fact I wasted a lot of time in the beginning doing exactly that!
HOWEVER in spite of this when listening to music it is incredible how the bass always has some sort of definite location or character. What makes it so incredible is this happens even with bass notes that seem to have no sharp transient or leading edge. Like a big drum whack, it makes sense your ears are cued to a location by the initial whack on the skin of the drum. Or pluck string bass, same thing, the initial string plucking off the fingertip. But all kinds of bass does this too. It SEEMS to be localized BUT it is NOT and the proof is I can run the amps mono and STILL hear the exact same localized bass!
None of this makes any sense other than our hearing does not localize very low frequency bass but does localize higher bass AND THEN combines the one with the other to form a clear image of where the whole thing is coming from.
In other words, a great example of psycho-acoustics.
