Best Subwoofer for $500?

I'm currently running an all Energy Connoisseur HT, and need a sub. The mains are C-9s, so there is some bass there, but a sub is always a necessary requirement. My room is only 12' X 15' with 12' cathedral ceilings. I have hardwood floors as well. I don't think I need more than a 12" in this room, and may be able to get away with a 10". I'm worrying about overpowering such a small area. What's the best subwoofer to fit my situation?