I am in the same boat as you, but can spend $100 more.
I'll be getting a SVS or HSU.
I've heard that Klipsh, and was not impressed. I can't remember why I didnt like it though. I listened to a bunch of different subs that day.
If I had to go cheaper, and could not do a cheaper or used SVS or a HSU, I would probably do a Dayton DIY kit.