Two Subs?

Two or more subs are mostly for balancing bass levels more smoothly across a room, not so much to get more or better bass at any one particular spot, though it may well help achieve properly balanced bass levels more easily there as well.

Duke from Audiokinesis is a proponent of using more bass modules for this purpose of helping to randomize bass modes throughout the room better. Of course, any pair of larger full range speakers do this to some extent, but best choice of placement for bass may not always be the best overall.

AK planetarium systems use 4 separate "bass modules" to help achieve this for example. This makes a lot of sense to me and the AK Planetariums are high on my list of potential high value speaker systems (especially for more problematic rooms acoustically otherwise) I want to hear someday.

I've always found room modes to be an issue with a single sub, ie very noticeable by ear variations in bass levels at different room locations. Also I find that it is possible to address to my satisfaction as long as listening position stays the same. I am confident the more sophisticated and complex eq solutions have up side as well, but not critical for me personally. I would prefer to have two subs rather than one though to help smooth out things within the room better and would probably go down that route next time, though I could probably live without it quite fine just as well as I have managed to do in the past.