I have to agree that $500 is the starting point for good bass. This puts you in the market for subs from SVS, HSU, Outlaw and Rythmik. There are a few decent options below that point, but they are mainly smaller subs good for smaller spaces. The Outlaw M8 and HSU STF-2 are good options for a bedroom or small space, but I would try and save for something nicer.
Denon AVR-1713 | EMP E55Ti | EMP E56Ci | EMP E5Bi | BIC F-12