Mike,
for subs under $500 there are no better choices than these two sites below.
http://www.acoustic-visions.com/~acoustic/products/subwoofers/pre_finished/

http://www.adireaudio.com/home_audio/loudspeakers/exact_series/rava.htm

I built my sub from scratch using MDF board, Adire Audio’s Shiva and plate amp from Partsexpress. I couldn’t be happier with the result especially knowing that I got at least $1,200 performance out of $425. Sure, it took time to do it but you don’t have to if you get one of those kit subwoofers. You can spend $500 on a store bought commercial brand sub but you won’t get the kind of performance Shiva (and other Adire Audio drivers) can give. Why bother with B-stock or used? Get yourself a brand new one. It can shake the house for HT and yet rolls out clean and sweet base for music. I’m saying this after comparing mine to a friend’s $1,000 Klipsch sub. My Shiva pretty much splayed that fine Klipsch.