Just for giggles I tried a 6CG7 in place of the 12AU7. Not bad. I haven't decided the best way to convert the board to use it though. I think the simplest method would be to cut the traces for pins 4, 5, and 9 and then run a twisted pair of wires from 8 and 9 on the J1 terminal strip to pins 4 and 5 on the board where the 12AU7 would be.
I just bring this up cause I've got a lot of 6CG7's.
I suppose it could be possible to make a switch to toggle between 12AU7 heaters and 6CG7 heaters.
Another option that may be less destructive would be to remove the socket from the board and wire in a chassis mount socket where pins 4 and 5 would go to pin 4 on the socket and then pin 9 would go to pin 5 on the socket.