I have tried MANY subs, and finally tried a Rel (B1), and it's absolutely the best all around sub that I have had. It does 2 channel excellently, and with it's proprietary input options (both high and low level inputs at the same time) handles Home Theater like a champ.
I think the Vandersteen 2Wq was the best 2 channel sub as far as integration, but the Rel goes much deeper, and is just as fast.
The JL Audio F113 and Velodyne DD15 (with the SVS PC Ultra 13 right behind) were the best Home Theater subs I used, but they were limited on hook up options compared to the Rel's for 2 channel.
Rel is my recommendation, the higher up the food chain you can go, the better.