I remembered a few more from my own gig days:

What do you call a good looking woman out with a bassman?
"Mother."

What did the drummer get on his IQ test?
Drool.

What do you call a guitar player who only knows two chords?
A music critic

Definition of string bending: what guitar players do when they can't find the note.

How do you keep two bagpipers in tune?
Shoot one of them.

What's the difference between a rooster and a trombone player?
The rooster may have a gig next weekend.