Agree with Mr Tennis. You are also rolling tubes with different gain factors which at first may give the impression of a dramtic change, but after time passes, that change may not sound as appealing, in fact you may even return to your original configuration because it offered something appealing that's just missing with the new configuration. Tube rolling to different brands is one thing, but to roll out to a differnt type completely is a whole different ballgame. I think that manufacturers do in fact employ complimentary tubes in their individual units. However, your idea begs the question, what do you do when two or three different tubes are excellent compliments with each other, but your piece of equipment can not use them in combination?? Are you suggesting that manufacturers build their equipment with circuitry to accept whichever tube you might have a fancy for trying, which would be very interesting, if it were possible!