I've sometimes thought about getting two of the same amps built so that I could more easily compare tubes. My preamp can drive multiple amps at the same time. I think only on here that would seem reasonable?
I use this same preamp to compare the same recordings (playing at the same time) via my DAC and turntable. It makes for some very interesting comparisons.
My situation is not exactly the same as the casual listener and I often have two of the same headphone amps around of my own design. I also do not use hybrid amps, I want to hear tubes and only tubes. Sometimes within hybrid amps and pre-amps, a tube is added as a buffer, a way of imparting some tube sound but not enough of it to tell a lot of difference between tubes. I have used identical Bottlehead Cracks, as well as my amp, the Incubus Elegan that was designed for pure tube listening. I also built a box so that I can have RCA in and 1/4 inch female out so that when I hook to a switchbox I can use the same headphones, push one button and be listening to the same track on another amp, or three other amps, I generally do two or four myself.
It is not only here that hearing two amps seems reasonable, my audiophile group uses multiple amps and compares multiple DACS, and power amps all the time cause we want to get opinions from multiple people.
That does not mean I do not do my own comparisons at home, there are single and double triode tubes I personally test all the time. I am always curious though how my thoughts stack up against a larger group that includes some highly qualified people. Because of some of my work I have tested most every type of wire made that could feasibly be used for headphone cables and interconnects. I have also modified my own speakers and headphones to get the sound I wanted.
It is easy enough to hear one tube or tubes, score a tube, hear another, then do the same. Personally I do better when I can do a direct comparison going back and forth between specific tracks to rate the tube on 25 specifics. Even the most subjective things can be made somewhat objective.
This is the setup I sent Schiit audio so they could use four pre-amps operating from one source. The signal is split four ways so it is identical going into each pre-amp, I built all cables and two of the three boxes. The final result could be one pair of headphones or one power amp and one pair of speakers. All tubes were concealed for a blind listening. Four tubes and a switchbox to hear all four.
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I should mention that
@bcowen was gracious enough to send me parts and assisted in building some extension sockets they used. Bill had heard some of the test tubes and his ability to spot differences is remarkable.