Monoprice SW-12 12" 400 Watt RMS (600 Watt Peak) Powered Subwoofer
If you are interested in a cheap budget DIY sub, you can do two 10" GRS subs in an isobaric configuration to get you a good response down to 20hz. You could pair it with any amp from 250-500w, like the Yung 500w. The disadvantage of something like this, is it would be bigger.
Thanks for the advice. I think I'll give it a try.