Well it really comes down to what kind of bass you are looking for, and what your price is. Some people want to feel a thump in their chest, others may just want some to balance out their sound and give movies a little more dynamics. The Klipsch RW-12D is regarded by many as the best $300 sub you can buy (new). It is a good performer and usually on sale for $299 on Newegg once a month. Stepping up, the new SVS PB-1000 is a great sub for $500, as well as the VTF line from HSU Research.
I would say given your setup and room size, that Klipsch is probably ideal in terms of value and performance.
Samsung PN60F5300 | Denon AVR-1713 | EMP E55Ti | EMP E56Ci | EMP E5Bi | Outlaw Ultra-X12