It's hard to go wrong with Hsu Research. They make some great subs in your price range. SVS makes very good subs, too. I think there's a link to them from AH. Velodyne is a solid brand, as is Def Tech & Paradigm. Axiom & Outlaw make nice subs, the latter being designed as a collaboration with Dr. Hsu. Lastly, the Rocket line by AV123 are stunning looking subs that perform well, and the cabinetry to incredible at the price.
I wouldn't rule out buying used, either.