Because these are different tubes and not really interchangeable it's hard to give you an answer. The 12au7 has a gain of about 19 and the 12at7 is 70 I believe. You would need to change the biasing of the tube to get it to perform to it's best. You could plug an au7 into at7 position and it should operate but not at it's best because of what I mentioned. 12at7 are often found as phase inverter/splitters. I commonly see 12au7 or 12ax7 as the preamp tubes in the first stage of amplification. Can't give you an exact reason why. I have used amps and preamps with both and it came down more to the brand of tube more than what kind of tube for me. "referring to these two tubes" Some people like the 6SN7/6SL7 family better and don't like either. I like all mentioned above. Don't know if I have helped you?
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