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

Variables.  Different speaker cable, different positioning, not giving your speakers enough room to breathe...  one pair might sound better closer to the wall, wider spacing, less toe in...  
possible negative results due to one pair moving the drivers on the pair not in use?

@b_limo Thanks for giving a reason and not just an opinion.
@tablejockey I've been instructed not to turn tube amps off and then immediately back on but to give them a couple minutes. Folks have reported burning out rectifiers doing this.

You are correct about the amps.  Five minutes at minimum is a good starting point.  

@hilde45  "I have a tube amp that requires shut down, short break, between speaker changes"

@tablejockey "What is the reason for the shut down?"

OP runs tube amps.  Not a good idea to hot-disconnect and operate a valve/tube amplifier without a load. Without a load on the secondary, the output transformer primary presents a high impedance load to the output tubes, resulting in higher-than-normal voltages that can damage the tubes and/or transformer. Damage is likely if you operate the amplifier with a large input signal but no load. Might get away with it if there is no input signal and not recommended for most tube amps.  

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.