How do you A/B amps in your system?

Yes - quite simply I reconnect the sets of cables (input ICs & out SCs) between 2 amps. Move the amps themselves, if space requires. You swap in an amp, and then listen AS NORMAL to your preferred music material, for a duration. Rinse, repeat. For an experienced audiophile, the sonic differences (more importantly - your interpretation of them) will at first manifest, then solidify / stabilize, and finally become quite consistent over time.

Switchboxes meant to reduce swap-time is a false optimization. Such scenarios do 2 things:

1. They fatigue or outright shut-off the portion of our brain that derives pleasure from music listening. This part is crucial to our subjective interpretation and overall listening experience. 
2. Fast AB comparisons further engage the brain's logical center, with powerful pattern matching that says: "yep that sounds similar enough; it's the SAME". That's why fast-switch AB or ABX tests often find little to no difference.

Passive two-way speaker + amp switch. One click and A/B. Works great. Luxman makes a high end version. 

@ghdprentice has it right, extended listening for each amp will tell which one is the best for you and your system. And you are right, you have to let the gear settle in for a while before you can truly judge its character, switching things around constantly will not give accurate results. 
 

I just went through this process in the fall, and am now selling an amp I thought I would keep forever. 

Thank you all for sharing your experiences and what you have found useful. @toddalin thanks for sharing your design and the warning about tube amps (which is one of what I'm comparing). And I guess @zlone your experience shows that it's possible to hear well enough to overcome bias, which was one of my primary questions.