Multiple subs for the win.... More than one sub helps to even out the bass in the room. Yes, you get more potential volume, but the main reason for more than one sub is for more even bass within the room. I went from 1 "more than capable" sub to adding a 2nd virtually identical sub. The subwoofer response is SO much better having 2 subs. By that I mean that you can/will get "peaks" in some seats and "nulls" in others... Add a second sub and you start to lessen the peaks and bring up the nulls. Add a 3rd sub and it helps more, 4th sub, gets things even more even.
research shows that going from 1 sub to 2 subs is the biggest bang for your upgrade buck. The benefits of a 3rd and then 4th sub are still there, but the increase in benefit decreases as you go along.
M60s, VP180, VP150, QS8s, SVS 20-39PCi, HTPC, JVC RS45, Onkyo TX-NR709, Shakers