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
You will need an amp for it but for a couple of hundred you can get a used amp that would work for a sub. An Adcom 545 bridged would be perfect. You should check it out. The VMPS sub is very musical and plays down to 17hz. They aren't called the "Largers" for nothing. The HSU is also good but for the money that VMPS larger will do it all and then some.Final note, VMPS subs have had great reviews for 20 years and are known for having excellent bang for your hard earned money.
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.