How do you A/B amps in your system?

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.

Some tube amps will require "an extra layer of protection" in that the milliseconds that the relay is open during switching can do damage to them.

Not shown in my schematic, but added later, each speaker INPUT also has a switch that inserts a 261 ohm, 30 watt resistor that "stays with the amplifier" so that there is always a minimum of 261 ohms across the output, even when the relays are open.

The value was selected after discussion with on-line "tube people" to select a value high enough so as to have very little effect when playing though a set of speakers, but low enough to effectively "load" the amplifier. This would drop a 16 ohm load to about 15.1 ohms, or an 8 ohm load to about 7.8 ohms.

On another note! For those who "poo poo" A/B testing, consider this.

You can listen to each system as long as you desire. Maybe you don’t even want to hear the other system. So you are casually listening to one of your favorite tracks and you come across a passage and say to yourself, "That sounds a bit strident. I wonder if it is the cut, or a feature/flaw of the speaker." So you instantly switch to "B" and replay/continue the passage and listen again for a difference. And of course they need to be the same volume for a fair comparison.

YOU CAN’T DO THAT ANY OTHER WAY!