I am a subwoofer sevant, so let me help you avoid some of my mistakes. Buy a brand known for good subwoofers. I have owned 2 Klipsch and a Martin Logan sub, and while both of those companies make good speakers, there subs were pretty weak. If you want decent or better low end, you just have to spend 400-800+ dollars.
So you could really have two strategies....but two smaller subs, like the pb1000, which will help even out the bass w/in the room, or get one full on sub and work really hard on proper placement. In my theater, I have the unicorn, the EMPtek 10i10i, a dual 10 inch sub. Really good sub for the $350 or so I got it for, but not a heavy hitter like most of the subs listed in the last few posts to this thread. Now I run the EMP with an Outlaw LFM1. People warned against me trying to run those two subs...but they seem to complement each other really well. The EMP is accurate and powerful above 40 hz and the outlaw hits low and get loud! I just level matched them with my ears and set the gain so the Outlaw is slightly dominate, but not by much.
Anyway, enjoy those EMP speakers and keeps us posted on your thoughts!