NHT SW2P not enough?

Here are my specs:

Room size is about 12' x 25', give or take, but it's also open on one side (to the kitchen and hallway). Ceilings are 9 1/2 feet.

I've got lots of space in the room for a second sub, but not much up with the other gear -- my 51" TV, dual centers, L/R mains, and rack have taken up most of that 12' in width.

My main L/R speakers are NHT 2.3As, centers are a pair of 1.3As wired in parallel. They've been being driven by an older Denon receiver, but as of the end of this week they will be feeding off of a Rotel 985 MKII.

I don't listen at earthshattering levels, but when something explodes you should feel it . . . .

I originally had the sub located in the front right corner of the room, but I got horrible resonances from the walls of the room -- just that kind of humming that you feel going right through your sinuses ;-0). So I moved it out from both walls, and it now sounds much cleaner and tighter, just not enough slam for HT stuff.

I've heard lots of different things about two subs -- it's good, it's bad . . . . I'd certainly appreciate any opinions on that, too -- SW2Ps are certainly cheap enough used.

Thanks for all the opinions so far!!

Actually, as it turns out, the problem has taken care of itself. I hooked up the Rotel 985 to my 2.3A front speakers today, and miraculously an extra 40-50Hz of bass appeared! (Guess I should have expected that.) With the front speakers now putting out a healthy amount of low frequency energy, I have plenty of bass with just the one sub.

Thanks for all the suggestions, though!