Single vs. Dual Subs

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'. There are also some neat tricks for finding good locations although the whole process is a lot easier with a measurement mic and some software. You need to also make sure that the speakers are 'time aligned' which you can do by measuring and with some simple maths.

It depends on the room, the mains and most of all, fc and slope.

If fc and slope reach into the lower mid-bass, one sub is localizable and must be placed between the mains.

IMO, 2 subs are mandatory for 2 channel. 3+ is a crowd.

Richard Vandersteen told me the more subs the merrier

I use a pair of Velodyne HGS-15s in the corners of a 14’ X 19’ room with acoustic room correction by an SMS-1 to supplement KEF Reference 1s below 80 Hz @ 24 dB/octave. The result is excellent. I’ve tried a couple of high-pass filters, but they seem to take a bit of air out of the sound, perhaps a peculiarity of the sticking them between an Ayre preamp and amp with the zero feedback.

I feel a pair of subs help to develop the sound stage.  They can even clean up the mids.