I am a fairly new Crack builder/owner and have gone thru many amps, both SS and tubes..... the Crack is indeed a very special amp that really plays very well for me with a set of Beyer T90's. 250 Ohms.
I can't tell what the stock 12AU7 is, there aren't any discernible markings on it but it sounds really great to me. The power tube is the coke bottle RCA and I believe its a 6AS7G.
I bought a $12 12AU7 JJ Electronics from
www.boiaudioworks.com this is a place I have bought several tubes from great to deal with and great shipping and the tubes have always come to me well packed and fast, I think there in Tempe AZ. (note I have NO vested interest in them just an FYI and another source for tubes).
I have only listened for a few hours with the JJ but its absolutely a stellar performer, I have a Salk Demo CD that has some really extreme and well mastered flacs and they sound just marvelous...
I would NOT hesitate to use a JJ 12AU7, only $12 for a matched triode tube!
Waiting on my speedball upgrade!
All the best
Alex