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

Both approaches inform and have value.

@david_ten Yes, but comparing particular element, like bass or treble extension can result in choosing wrong gear.  In early seventies, before Transient Intermodulation Distortion surfaced, you could get a lot of amps with wonderful bass, treble etc, but sound of these amps was tiring over longer period of time.  When comparing such amp by switching, it would always win, but it was unpleasant to listen to on long run.

Running 2 systems myself. I build a switching unit with a break before make switch. It’s never a good idea to take a chance of accidentally connect 2 amps simultaneously. Something is going to give. Not hard to make and very cost effective. The switching is close to instantaneous. There are some switching units on the market you could try. Do not use speaker selector switches as the chance of having both amps connecting simultaneously is high in which case pop goes the weasel.  Break before make is the key phrase. 

@kijanki I’m in wholehearted agreement with you. I am also in wholehearted agreement with the "other" position and points made above.

I realize this (being in both 'camps' simultaneously) tends not to happen often in these environs.

I use both approaches. Each ’way’ can and does (though not always; and not always with certainty) offer unique and disparate information. This addresses non-overlapping areas of personal (human) interface and connection. Objectives for each ’approach’ focus on different natures / aspects.

Key (for me) is the overlay and the interlacing of the information gathered and how I use it to make a final decision and choice.

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

Well, no, that is not the case at all.

First of all what you suggest is impossible. Even instantaneous switching still the goal is comparing. With what? With what you heard. When? In the past. Where’s the past? In memory. Nowhere else.

You simply haven’t thought this one through. Not at all.

Which aspects of the sound? Well how do you even begin to answer that one? By thinking over the different aspects. Which involves what, again? Memory.

All you’re doing with this instantaneous switching malarkey is moving the proverbial runner halfway to the finish line. You know the story, right? To get to the finish line he must go halfway, then halfway, then half of that.... never gets there. When in reality, WHOOSH! Right on by.

That’s exactly what you’re doing. Inventing some nonexistent constraint, insisting its real. When in reality, WHOOSH! Right on by.

Besides, what about warm-up? Break-in? Acoustic treatments? What about differences between recordings? Do you now require duplicate identical turntables just to decide which LP is better?

Its really just beyond silly.

Especially since, if the difference isn’t big enough you can still be sure after the 5 minutes it takes to change something out, then why would you care anyway? Answer me that one.

I would have thought there was something practical on the market already.

Well, there would be. If it would serve any purpose. Which it doesn't. So its not.

But seriously, the question stands: If you can't be sure after a few minutes then why do you care?