Best Used Subwoofer for $500.00?

I need opinion's on accurate, tight, non boomy sub's, that can be had used for under $500.00 I would prefer front firing. B&W, Paradigm, Velodyne, Etc. Thank's Betelgeus
If you could come up with another $100. You can find Paradigm PW2200 for $600(used) sometimes (far and few between) This sub is VERY VERY tight and accurate. HARD HITTING 95+db@20hz!!(HIGH Q) The Velodyne CT-150 is +25% BOOOOMY -25% accurate. B&W 1000 is in the middle of the two, but sheer output is -20% of the previous 2. But all these subs are going to cost you another hundred I'm afraid. But the PARADIGM PW2200 in MY opinion would be my choice even if your budget was $900.00. I love accurate bass.
I am using a Definitive Tech PS200TL that was $550 (list) new. For a medium sized great room where I have my setup, it is tremendous. Low end on DVDs rumbles the glasses in the cupboards, and Gerald Veasley HDCD bass can really be felt. I would highly suggest this alternative.
Definately try to find a used REL Stratta II. In fact, I just saw one for $599.00, only one hundred above yr limit. Goodluck.
for sale at $500 at