How do you A/B amps in your system?

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).

I leave the main amp in place, put the new amp in the floor in front of it and hook it up.  I listen for multiple days unless it isn't really close.  I always do ABA unless it is obvious.  Somtimes ABABA.

Jerry