+1 rocray. I have an SVS SB2000. It's a sealed box design and not too big, and it reaches down to an honest 20Hz and feels every bit of it when called for. For $700 I think it's a bargain, but I haven't compared it to the others, which are also excellent. Best of luck.
Subwoofer suggestions
I have a pair of Pioneer S-1ex's , now looking to add a sub. Room size is 12x18 fairly well damped.