Correct me if I am wrong but I don't think you are supposed to deviate from stock supplied tube types (for instance substituting a 12 ax7 for a 12 au7, as they have different gains. Different brands of 12 au7s will give different sonic signatures. 12au7's have the lowest gain and lowest intrinsic distortion (compared to 12ax7s and 12at7s. To my ears, the Mullards are soft,warm, and lush sounding. Telefunkens are very neutral sounding, and detailed. The RCA's, and their premium variants, 5814s, and 6189's are kind of "in-between". The regular version Amperex tubes seem to me to have more "air" and top end detail than the others, and the premium Amperex 7316 versions are highly sought after (I have not heard them). My preference (IMHO) is for the Telefunkens, but your ears will tell you what you prefer,as, in your application, results may vary.
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