I was wondering the same thing myself as I was watching it this week! The original music in the show was composed by Schwartz, and he plays bass on a lot of his stuff on the show (and on others) according to his website (which has a link to our DB forums, by the way). I went looking after being intrigued as to who was playing the nice tribute to Jaco's 'Kuru' in the closing credits

One of the other tunes used on the show has echoes of 'If You Want Me To Stay' - clearly the guy has good taste

He also composed and played bass on the excellent theme from Northern Exposure.