Well, I had 12AU7 AMP before, and yes, a lot of them are just nothing but NOTHING, however really surprising me that nobody have mentioned Mullards, Telefunkens, Brimars, Philips, Siemens, Valvo, JJ Tesla, Amperex Holland made so far, how older>the better. In general look at NOS, means New Old Stock, so you have to find one you like it at most. Once again, I am coming from 12AU7=ECC82 tubes, so here is the link, which is pretty informatief
https://www.tubeworld.com/12ax7.htm
Personaly, I found Telefunkens and Mullards are the best, using with Grado's, however you may have different taste. Most of these arent cheap, especially Amperex. Also be careful with fake Chinese Telefunkens, so you may check this link as well
12AX7 Tubes in Stock
12AX7 / ECC83 / 7025 / CV4004 Tube Types
About the Amperex. These are pretty common on Ebay, but the thing is, they do not match every amp. Once I bought Amperex 7316, which seems to be one of the best around, but after I tried them, I went back to Telefunkens pretty fast. The presentation wasnt right, so before you spend a lot of money, read some info first. You can always send an e-mail to those who selling them and ask which amp they have use these, or ask the Dr.Lloyd, the maker of your amp, which 12AX7 you may use to get the best results.
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Good luck to you and I do hope you have some budget