My understanding is that the outer position is a voltage gain stage and the inside is cathode follower. 12AU7 is a lousy choice for a cathode follower. Its transconductance (gm) is far too low as well as its gain is low, this leads to a poor CF. The 6CG7/6FQ7 is a 9pin 6SN7 and has far more gm and much lower distortion (regardless of vintage).
Morgan Jones writes that the 12AU7 is surprisingly consistent regardless of brand or make....surprisingly poor (in terms of distortion measurements). I am paraphrasing, but he said something to that same affect, actually a bit more bold.
When it comes to this setup, I'd probably guess a 6CG7 for the gain stage and 6922 for the CF would be the bee's knees, but that depends on the SS stage and whether something with more distortion would be more pleasing. Synergy?
