How do you A/B amps in your system?

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.

Typically, after one quick switch to hear note any obvious differences. I’ll put the new amp in for a week and try not to "listen to the amp", just the music. Then I will switch. Do this a couple times and the differences in gestalt and details will become clear.

Rapid back and forth changes... with all the physical stuff in between will erase any memory you have of the music you heard before. Professional dealers have done enough A-B ing that they may have developed a better sense than most of us normal people. Besides there are dozens of aspects to the music... so you can only hold a couple during quick changes. Experienceing and learning the differences in amps requires time and lots of different musical types.

Like @ghdprentice, I use long term listening sessions for evaluation, not A/B back and forth.  Extended sessions give me chance to listen to a wide variety of music and get a good feel for it's strengths and weaknesses.  Some things really start to stand out after a few days.  

Thanks for the thoughts, and @lanx0003 thanks for the exact tool. Of course on Audiogon we will have to debate the sound of the switcher! Lol

It dawns on me that the advice to listen long also allows for the amp to settle into the system (which seems like voodoo but I always notice that it helps).