I am baffled on how just the Volume knob would have been sheered off, and none of the other controllers/knobs were damaged?
I would assume the knob was laying in the packaging? You might be able to use some sort of glue for metal, and glue the knob back on, not sure. Do you have any pictures so we can see what your up against?
This is to bad it happened to you, I'm sure you were trying to save a buck, not sure what you payed? Was it a lot less than the Factory Outlet price on a new one from Axiom?
You might be able to take off the amp plate, but you need special Star Torx screw drivers.