I second the Rhythmik recommendation. I have two 15" in my home theatre. They are fast, clean, tonally accurate and present no flab. They’re superb subs.
Subwoofers suggestion?
I don’t have small speakers and my speakers can do bass well, but years back, and with other speakers, I had a subwoofer and lingering memories make me occasionally think that a sub or two might do wonders in my system. (I owned a pair of Maggies MG2.6R’s and ran them with a Sunfire sub). My present speakers are Revel Studio 2’s. I’m aware that a pair of subs is a superior way to go but for cost reasons I’d likely buy one now and add a 2nd one down the road. Suggestions are cheerfully awaited!