Subwoofer recommendations for mostly music


I don't know anything about subwoofers, so be kind to me! I currently have some sort of Velodyne hooked up to my system. It doesn't seem to add a thing to my system - possibly because it's crap, or possibly because I don't have it set up correctly! I would like to get another subwoofer anyway, so would like some recommendations. It will be part of my HT system, but I really want one that is good for music. I listen to classical, classical rock, jazz and World Music for the most part.

I will be pairing it with bi-wired B&W CM4 speakers (center is a B&W CMC) and I use a Rotel RSX1055 receiver. (I know a lot of people here don't like B&W speakers or Rotel components, but please don't suggest that I upgrade this stuff first - I can't afford to and don't want to!)

Budget wise, I would prefer <$500, if possible - used is fine.
oakiris
I'm not sure if this thread is still being used but I came across it because of having the same issue as Oakiris - knowing nothing about subs and not sure if mine is working properly. I've owned the Onix UFW-10 for awhile but only recently hooked it up to my Cary SLI-80. I know I'm not supposed to "hear" the sub, but it just doesn't sound like it's adding much to the bottom end. I'm using it to complement my Silverline Sonatina II's. Any thoughts?
Look into having a custom made cabinet for the TC sounds LMS5400. It will destroy most of the overpriced stuff here.
Its an 18inch woofer that can run over 100db at 20hz with less than 5% thd.
Very linear.
Sub technology has changed alot since the start of this thread. Best subs for the money IMHO are:

Hsu
Sunfire
Martin Logan

In general, sealed subs sound smoother than ported subs. Avoid VMPS.