My experience is reflected in much of AGAssarsson response.

I'd just like to add that it's really difficult, if not impossible, to get a sub(s) to sound its best in most rooms without some EQ. With 2 subs EQ'ed the low frequencies really envelope you and give a better sense of the total sound field enveloping you as well. It's really an uncanny experience.
The subs do not have to be in the same spot or near the mains for accurate phase response in multichannel, that's what the speaker distance setting is for.
Also, running 2 subs will generally provide bass with less distortion, as you don't have to push 2 as hard as 1 to get the same bass level. Less demanding on the sub amps as well.

I recommend trying 2 subs. With some patience, no, a lot of patience, they can make a dramatic difference in providing clean bass and a more 3D soundstage.

There is an excellent articel on the web by Dr. Harmon of Harmon International that describes via computer modeling the effects of subs, from one to I think 400! in a room. They concluded that 4 are ideal but 2 get you pretty close to ideal.It's worth a read.