If this thread is still alive . . .
pma, I'm currently in the middle of making some simple A/B switching gear for a stereo amplifier comparison of my own - on a much smaller scale, just a cheap class D ($40 or so Amazon) versus a tiny desktop amp, the class A/B AudioEngine N22. I am also going to compare both these guys to my elderly but still pleasant-sounding Kenwood KR-4070. I won't be doing any waveform analysis or scoping or whatever - just using my aging, highly unreliable ears. I guess you would say it's a DIY project for fun, done at a crude level.
So far I have just built a mechanical switch for the amp/speaker connections. Works fine if I sit close enough to the amp setup - puts me right in the magic triangle at moderate to low volumes. I may go to relays just because it would expand my seating & listening options.
However what I am including, but what I don't seem to see in your setups, is switching over the input source to the amplifiers at the same time that I switch the speakers. I want the same exact input in other words - whether it be my iPad with its built-in DAC or my Mac M1 Mini connected to either a Schiit Modi 3 DAC or a Focusrite Scarlett 2i4 DAC. Thus I have an additional switch that has stereo RCA input jacks & swaps a pair of RCA stereo outputs.
The other option I can think of is to not bother with a switch for changing the inputs, but simply manually changing the input. This is easy to do given that aside from simply going straight from a computer or tablet 1/8" plug output, there are RCA-to-1/8" cables to make it quick and easy even with a snobby DAC like the Schiit which offers only RCA outs.
So anyway I don't see anything like an input switch in your design. Are you doing the input switch manually, which would suggest that you listen to fairly lengthy segments of music in your A/B'ing, rather than doing quick switching from one to the other? I don't trust my own memory to handle lengthy listening before I switch over - too likely I will fool myself. Or do you have an input switch somewhere & it's blindingly obvious and I'm just missing it?? I'm no sort of audiophile, just a DIY middlebrow, and quite often I do in fact miss the obvious. So illuminate me if you would - thanks.