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
Thank you all for your responses. I will check out all of the suggestions.

I forgot to check last night to see which Velodyne sub I have - I bought it at Circuit City so, not to stereotype a big box store or anything, I am assuming it wasn't hi-fi quality.

Then again, perhaps Newbee and others are right - I am expecting a great sonic difference when using the subwoofer, but there really shouldn't be one as the sub should just blend in with the system.
I finally checked my current subwoofer to see what it is. It's a Velodyne CHT-8. Now you know why I want to replace it!!
a really good way to check out a sub... take the soundtrack to "mission impossible" with and without sub, a bad sub will obviously not blend seamlessly, whereas something like a Rel Stentor will make you sell firstborn to own.
At that price range: HSU VTF-2 (not the STF) the HSU VTF 2..hard to beat at $500