Yeah, what Peter said.
I have the Hsu STF-2. I love it. Tight bass, loss distortion. I find that it goes plenty low. Could it go lower? Sure, according to specs. Could I tell if it went lower? Maybe not, especially considering my wife's dislike for wall shaking bass and consideration of neighbors.
If I were to do it all over again, I would have considered the Axiom line. A few years ago, their subs were typically eschewed for the Hsu and the SVS subs for home theater. Times have changed, though. If you have the extra cash, the consensus is that the Axiom's are pretty hard to touch.
In fact, I think the addition of the EP500/600 contributed in no insignificant way to Axiom's recent best in show award at CES. Apparently, they really do thump with precision.
To be honest, though, I think you'll be happy with whatever you get. When you start talking about the differences in comparable models in teh Hsu, SVS and Axiom subs, the differences get to be very minimal, from what I understand.
