Amplifier and Speaker A/B switcher

What is your equipment? There is a place to show photos and ID you equipment under virtual systems. This will help a lot in answering your question.

It depends on your equipment. The better it is, the less likely there is such a thing. I have tried a couple of very high quality (one was Rotel ((?)) ), (very heavy duty mechanical switchers and the degraded the quality significantly. 

cayin tube amp and schiit aegir amp

what about just a speaker switcher to switch one pair of speakers between two amps? would that avoid some degradation?

Depending on the concerning ear, everything you put into the signal path will degrade the sound.

Years ago, I needed to do what you want.   But I needed to be able to switch 4 turntables, 6 amps/receivers, 2 aux and 12 pairs of speakers. Plus, spares of each.  I had a hard time finding the switches i needed.   And wiring them together would be a rats nest pain.  That's why you pay the big bucks. 

So, I made a Patch Board.  Pamona banana jacks for power outs and speakers.  RCAs for turntable, aux, pre out and amp in.  Using patch cords I could switch. ab, mix every which way.  Was not a quick turn or push of a switch but did the job.  And all the connections were right in front of me.