I agree with Mulveling. 12BH7 is another tube type that is said to be a good sub for the 12AU7, which is to say, an upgrade.
Yogiboy said "5914, 5963,6189 and ECC82 are all replacements for the 12AU7 tube". ECC82 is definitely only a 12AU7 by another name. In the case of "5914" in Yogi's quote, I think that's a typo and he meant 5814. The open question is whether 5814 is only yet a third name for a 12AU7, i.e., the same tube with a different alphanumeric code, or actually a different tube electrically that can be freely substituted for a 12AU7/ECC82. As to the 5963 and 6189, all bets are off so far as I am concerned but one can look up the specs in any good tube manual to see whether they are identical to 12AU7/ECC82. I'd preferentially go with a 6CG7, provided a 6.3V filament supply and the ability to reconfigure the socket or with the 12FQ7, provided a 12V filament supply, that requires no soldering. But both of those latter tubes may require a bit more filament current than the 12AU7/ECC82. Most gear can handle it. And I feel very strongly that the sound will improve.