Have you ever used a separate speaker selector unit to audition speakers? Would you?

Yeah. Don't.

Nonsense. Or maybe true if you are using a cheap switch. Niles, anyone?

A good selector switch is really a set of time adjusted relays that have little or no effect on the signal pass through them. I own and have used a Luxman AS-50R selector w/ remote for years and cannot hear it in the system. Proper placement and wiring is crucial. I think I paid just under $200 plus shipping from Japan on eBay several years ago.

Luxman makes several models, all good, but it's a lot easier and less disruptive to switch from your listening position. Van Alstine also makes a very good unit (ABX Switch Comparator).

These two companies also make good unbalanced (RCA) line level switch boxes. And Manley is making the Skipjack line level switch again, as I recall.

Look, no one would put a switch box in the signal path for no reason. But if you need to compare gear it's a much better way than powering up and down and swapping cables.

For those of you with a myriad of arguments against using a GOOD speaker selector I say great, don't use one. I find it almost essential for making A/B comparisons. No, there is no free lunch so you have to do some work, like figuring out how to level match each and every pair of speakers.

I don't think I or anyone ever said that a speaker selector was the ONLY method that should be used. Every audio component requires a long-term test. But, audio memory being what it is (generally poor, despite what you may believe) this is the only way to hear glaring changes in their raw state. Is that valid? Sure, as one data point.

Are selector switches a weak link? Maybe, but I don't think that a GOOD selector box color the sound. At least I, my former customers and distributors never mentioned it if they did. But as @ aburnhamuu45 mentioned above if both speakers are wired through the same selector what's the difference? You are evaluating the delta between two equalized sources. Your long term test (without the selector) will provide your sound baseline evaluation.