The problem I've run into with simple circuit AB boxes is ground loops and hum. It's so irritating that you may be better off buying something with isolated transformers and ground lifts. The best is a Lehle Amp Switcher, but they're expensive.
Check into the new George Lynch Morley Tripler. For $99 you can switch between 3 amps, or combinations. Plus each channel has a trim pot. It may be what you need.
