If you are looking $500 internet direct range, best bang for buck is Hsu IMO.
I owned this. Blew my klipsch sub-12 it replaced away.
http://www.hsuresearch.com/products/vtf-2mk4.html
Cool, it is a nice sub. Only reason I sold it is my room is 5300 cuft and I needed more output. My friend is using in his living room and it bumps very well.