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

I built my own switcher using multiple speaker relay paralleled to minimize contact wear causing distortion.

Or use a abx-switch-comparator

Problematic if you are talking about tube gear that doesn't like to be on when no speakers are connected, but no problem with solid state.

Just have a friend hook up one at a time - a swap of interconnects from one to the other (when muted, of course) takes a minute - turn them off if worried about it.

Listening blind and not knowing which is on is the acid test of whether you are really hearing a difference or imagining one based on cost, expectation etc.

Problematic if you are talking about tube gear that doesn't like to be on when no speakers are connected, but no problem with solid state.

Connected to a dummy load resistor before disconnecting speaker from a tube amp can solve the problem, and no need to power off the tube amp to eliminate warm-up time.

I believe that the mind forgets what it heard 5 mins ago and internal biases kick in strong.

Yeah, my head was once stuffed full of this exact same nonsense. True story, I drove 200 miles to a store to compare interconnects just because they had this device that allowed you to switch back and forth instantly. Yeah. So I drove 200 miles to hear for myself.

Only when I got there, owner says sorry, we had to send it back. But you're welcome to compare anyway. And showed me the amp and let me alone.

So I listen to his interconnect a few minutes. Okay. Whatever. Flip the source. Swap interconnects. Flip the source. And.... WTF!!!! I mean WTF!!! Thought for sure I had messed up this guys beautiful expensive tube amp. Could not cut it off fast enough. What the.....?!?!? Everything hooked up right. Well then.... what?

Gradually it dawns on me. The freebie patch cord I had brought along to compare sounded so awful it was obvious instantly and from behind the speakers. Turned it back on, sat down, forced myself to endure what I had been playing and enjoying every night for years. Could. Not. Believe. It.

But hey, if you believe you forget, I believe you. Can only imagine how many fights you must get into with your wife, never being able to recognize her voice on the phone, but hey, I believe you. Really. I mean, that is after all why they have Caller ID, right?

Hahaha.. Pretty funny and I'm not saying you "forget" a blatantly obvious sound difference. We're not playing in that field here. Audiophiles are obsessed with the slightest nuances and to believe that your superior hearing memory is immune to certain biases is naive.

Everyone knows it's BY FAR better to A/B test with instantaneous switching than relying on memory and what you think you heard.  

Thanks for the other replies.  Unfortunately, I'm not up for building my own.  I would have thought there was something practical on the market already.