I despise A/B testing, but it has saved me a lot of money. But because I am so subject to expectation bias, I only do so blind (speakers not included). I found this device that allows me to do blind testing as all of my equipment is in the room just behind my front wall. It has a button to switch between the two inputs and another individual pushes the button so I can not see what I am listening to. Is this box perfectly transparent? Probably not but then neither is anything else. And I have gotten to the point where if the differences between two components is masked by whatever lack of transparency is in this box, I wouldn't buy it anyway.
It works for anything that uses XLR cables (DAC's, preamps, amps, etc)
I discovered this little box when I was looking for a way to (blindly) compare two 7.1 SSP's so assembled a "Rube Goldberg" device consisting of 4 of them and a spring-loaded bar that would activate all of them at the same time. Worked like a champ. (This device is now for sale should anyone be interested. Cost of parts was just over $600). The unpainted wood part is the base and the black handle is the spring loaded push bar.