It’s pretty easy for me. My rack is between my speakers and all are quite a few feet from the wall behind the speakers ("front wall").
Each amp has speaker wires already in the back.
Let’s say I’m listening to A, first.
I play a test tone (say, 250 hz.) and do a db reading on my phone. (e.g. 80 db on a C weighting).
I listen to amp A.
SWITCH: To switch from A to B, I
- turn off A
- move the preamp cables from A to B, and
- switch the speaker cables from A to B.
- then I turn on amp B.
I play that same test tone (250 hz.) and do a db reading on my phone and match it to amp A using the preamp volume.
I listen to amp B.
The switch between amps takes maybe 2 minutes.