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
I just installed (and am still tuning) a REL Q108. It made my Von Schweikert VR-1s even more perfect. I set up a small listening room which is 13' wide the back to the speakers, 10' deep (my chair backs up to the wall), and 13' tall. The REL sits in the front left corner out of the way. I loved the VR-1s for their soundstage and clarity but with the lowest frquencies (below 35 Hz) reproduced by the REL, they are perfect. I don't really know its there, except that it seems the bookshelf monitors now how a fuller bass. Not bad for $750! I think the REL line is great for just music. Pick the one that pressurizes your particular room. Good luck!