Ok, I know I’m probably driving some of you crazy (feel free to slap me if we ever meet), but I wanted to throw one more option into the ring.
Axiom EP350
It’s a little more powerful than the Hsu and it goes to 18Hz. And it’s smaller by about 3 inches, which would actually be helpful. It is about $250 more, though. I know Axiom isn’t a "known" subwoofer maker, but if their subs are anything like their other speakers, I can’t imagine I wouldn’t be impressed.
Review