I play in a trio and usually switch to a different pickup for a solo. I have separate volume controls for the two humbuckers on my guitar so I'll generally turn the neck down a little bit, and leave the bridge cranked up. That way, when I go to the bridge for the solo, it's already louder. I think there are modifications you can do to the wiring of your guitar that allow you to use a knob on the guitar as the master vol knob on the amp, so to speak. You wouldn't lose any of the 'crankedness' of the sound this way. I think Jerry Garcia had something like this.

Eben