I found the Belkin unit to be fine for bass and treble - far better than the Blackberry unit I bought first (and returned). I don't think that the Belkin is as good as original quality CDs. I think that I'd have a hard time distinquishing the difference between the belkin over bluetooth or wired.... but I can believe that wired is a touch better. From my listening, I don't think that the bass suffers at all though.
The Blackberry wasn't nearly as good. It was noticeably poorer quality over bluetooth (although I liked the unit itself - and how it integrated with the VR in the car.)