How do you A/B amps in your system?

@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.

I use a Beresford switch (from England). Allows me to switch from my Tube amps to my solid state amp. It has the added advantage of having dummy loads in it to prevent switching to tube amps without having the speaker load connected.

I bought an Audio by Van Alstine ABX Comparator to switch between my tube and solid state amps on the fly. Works like a charm  - it allows one to switch between 2 amps, 2 source devices or preamps, and 3 sets of speakers, in any combination. It also has an automated switching mode for true blind comparisons. Highly recommended.

I have a Schiit Kara preamp. It has three gain settings including a passive mode where the signal goes only through the RL volume control, and if the volume is wide open, not even that.

There are both SE and balanced inputs and outputs. I can connect multiple preamps, DACs, CD transport, etc. and output to multiple amps or powered speakers.

Selecting the correct input, gain mode, and volume setting I can quickly switch which combination of gear I'm listening to. It's very useful for quick comparisons though I do agree that extended listening is critical to a full evaluation.