How do you A/B amps in your system?

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. 

Thanks, @mashif, I also have a Schiit Freya preamp available -- which has multiple pre-out options. It would help with allowing keeping input connections into different amps and remove one element from switching, requiring just the speaker cable changes.👍🏻

@ toddalin thanks so much for bringing the topic up of tube amplifiers. Your design/ piece of equipment looks pretty intense. For everyone else there is a guy out of England that made me a loaded switcher box because I have a tube amp as well as a set of hybrid mono-blocks that I felt needed the extra protection and I didn't want to fiddlle-faddle around with switching cables around. I think the idea of comparison comes down to your ears and what sounds "right" for you. You can do all sorts of tests such as have been mentioned but in the end it has to "sound right" for you, your ears.