I need a musical sub under 500,,,,new or used.

I'm currently using a velodyne ct-150 for music. It is extremely slow. It's great for movie impact but it just can't keep up with the music. I would like to get a musical sub but I can't afford the megabucks sub (rel/sunfire, etc). Looking to spend about 500 new or used. Any suggestions on which sub I should be looking for.
B&W ASW 600 sounds good to me. 10 inch 150watts. $500. I'm fairly new to the high-end stereo game but this is what I own and it's nice and tight for music. Some of the others that I listened to(including HSU) sounded way to boomy for me. I have a pair of B&W 602's and the sub blends in perfect with it. You can't tell that it is there.
VMPS New larger subwoofer..it sells for 700$..I don't think you can find a used one..no one sells them on the used market..why?/because they are so good.....
Hsu distributed by Ascend Acoustics or Parts Express sells a sub. These would be two options.
Lots of suggestions so far. I'd just add, at this price level, avoid anything with a woofer more than 10" in diameter. The amps in the cheap subs and even in some more expensive ones (like your 15" Velodyne) are not capable of controlling that woofer cone. They are "slow" and sloppy, as you have heard.
Find a sub with a 10" or two 8" woofers made by a company that also makes good speakers for music.
You didn't say what speakers you're using, so I can't say where to set the crossover. But, from what I've seen, a lot of people have their subs set way too high in the frequency range, over-lapping the performance of their main speakers, and that's why they sound so bad.