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 have read that passive subs can be better than active subs, since the enclosure doesn't contain the potential interference from an internal amp. But, I really don't have the room for another piece of equipment (another amp for the sub) so that is why I have been looking for an active subwoofer.

In case I change my mind, though, what amp to you recommend for a sub?? :-)
For music the Force.

The Onix UFW-10 is another really good for music sealed sub you should look at but it only gets down to 25Hz.
I really like what I have read about the Rocket Onix UFW-10. Great price and good looks, too! How important is it for a sub to be able to go below 25Hz?
Well, after all of this, and everyone's recommendations, I just bought the Onix UFW10. Your fault, newbie13. (j/k) I have spent the morning reading reviews by users, etc., and the Rocket really sounds like what I want. What finally tipped the balance, is that the company is based here in Colorado and I can pick up the sub from them tomorrow!

Thank you to all of you who took the time to help me with this decision.
I've a Velodyne DD-18 now after replacing a Velodyne DD-12. It does (as did the DD-12) wonders for anchoring my B&W CDM 9NT's... I listen to vinyl quite a bit and wouldn't think of turning the Velodyne off! I find it hard to understand how yours doesn't add some magic although you didn't put the model in your post. The Digital Drive series is OUSTANDING... and the 12" DD-12 offered me all I could want. The move to the 18" driver wasn't only done as I like what the 12" did so much I figure more of a good thing is a good thing!