Amplifier and Speaker A/B switcher

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.

i think i'm just going to swap the speaker cables by hand between the two amps for now. hopefully the banana plugs can handle it daily. i don't want to lose any sound quality.

still open to any great ideas or good devices for this

"swapping the speaker cables by hand" That's the way most of us do it.

What about turning the amps around so the speaker connection are facing outward?

How often do will you be doing this?

If I had to do it again, I.would still go with a box, a bunch of dual banana jacks and a pair of dual banana jumper cables.  I don't think there is a switch that is as neutral transparent as a banana plug or screw down connection.  High current solid state switch?  Works for line level switching.  I'll throw this one at McGowen.

I used to demo 3 amps: Fisher 30 wpc Tube Mono Blocks 80az; Fisher 35 wpc 500C stereo tube receiver; McIntosh 250 wpc SS MC 2250 stereo amp.

Start with some hooks behind each speaker at 'reachable height'

Cable from each speaker with female banana long enough to hang on a hook behind each speaker.

3 sets of speaker wires (1 set/amp). I wrapped each set of speaker wires with different color electrical tape at each end to know which amp.

With male banana plugs that have tightening pins (also hanging from 'reachable hooks'. They insert loose, then tighten the pin. Tightening because frequent in/out not good for regular banana plugs

https://www.parts-express.com/Stacking-Banana-Plug-with-Dual-Set-Screws-Poly-Carbonate-Sh-091-3608?quantity=1

Lastly, you need to make A,B,C pencil marks on your volume control so they are 'matched' volume when comparing.

Same thing comparing cartridges, pencil marks to match volume.