Solid state or Tubes preamp phono stage.

The problem with tube phono stages or any tube equipment is the tubes. There is significant variability in even the same brand so unless the tubes are being tested you have no idea what you are going to get. Tubes self destruct eventually and you never know when they are going to do it. People get psychotic with tube rolling. Tubes are a PITA. Do they ever sound better? On rare occasions yes, in most no and this is coming from a guy who has lived with a tube phono stage for 30 years. I also just bought $46,000 tube amps and there are 20 output tubes in each amp, a glutton for punishment for sure and I am very apprehensive about it. Many people have the opinion these amps are the best at driving my speakers. I certainly hope this is true because I know for a fact JC1+s do a fabulous job and they are 1/2 the price. 

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.

Yes that was a typo error. BTW, my tube power amps use 12BH7 tubes and I was told from the manufacturer that I can sub a 12AU7 in place of a 12BH7.

It should be okay to try. The 12bh7 supposedly draws more heater current, but they should otherwise be pretty similar as far as compatibility. So if the circuit can handle a 12bh7, a 12au7 should be fine.

But the 12BH7 will sound better, by all accounts (see TubeDIY Asylum).  I haven't tried that type myself. Now, if the 12AU7 is being used as a cathode follower, then it does not do so much sonic damage and can probably be tolerated as is. In my own search for the "best" 12AU7, I ended up with genuine Amperex Bugleboys, because I had a few in my stash, purchased new by me in the early 80s. But even those were bested by the 12FQ7.