Best way to A / B test amps? Use a switch?

I have two systems I want to do some a / b testing.  Instead of believe what you tell me I should hear, I want to see for myself.

I believe that the mind forgets what it heard 5 mins ago and internal biases kick in strong.  Therefore, I'd like to be able to immediately switch between two amps or two preamps immediately.

Any thoughts on how to do this without damaging one of the systems?