Moon,
Partly the answer is simply "2 subs" because that allows asymmetric placement in order to minimize room effects - as noted above.
However, there's also this:
If you listen LOUD to LOW frequencies, 2 is also better because all subs behave worse at low frequency as SPL from that sub goes up. If you're listening at a given SPL, each of your 2 subs is playing 3db to 5db lower than a single sub of the same design. Let's say that SPL is 100db. Depending on which specific sub you're using, you will see markedly superior performance (distortion) at some particular frequency (range) where the sub is comfortable at 95-97 db but stressed at 100db. For many subs, this may be as high as 50+ hz, for others, it's as low as 20hz.
When a sub gets uncomfortable, it tends to fall apart pretty quickly - see subwoofer tests at HTshack.com's forums for "subwoofer tests".
So, listening levels, frequency content of the program material, room size/placement (determines actual output SPL from sub needed to achieve desired SPL at listening location), and particular model sub are all important in answering the question. In a small room, at 85db peaks, most music will not stress a decent single sub. At 105db peaks in a large room, HT content will stress a pair of excellent subs.
IME, 2 is MUCH better. Some people would suggest that all of this is usually inaudible, so YMMV.
Good Luck
Marty
