I'd suggest emailing Mark or Nick at Rogue and asking, they are excellent in responding to questions. Also, if my old man memory is working I think there was a thread sometime back that may actually contain a recommendation that Mark had made, though it may not have been with Sylvanias in the 12au7 positions.
Rogue Cronus Magnum preamp tube
After listening to my Rogue Audio Cronus Magnum for a few weeks I changed the center 12au7 preamp tube from the stock Sylvania to a Telefunken ribbed plate. I got more of the detail I was looking for but a bit too much. Is there a tube that will give me a sound somewhere in between; retaining the warmth and lushness of the Sylvania but adding in a bit more detail?