The GE Essential Characteristics Handbook and Smith Tube substitution book say 12 AU 7 can be replaced with a 12 AX 7. (NOT 12 AT 7 with 12 AU 7)
I have never tried that experiment . Before plugging anything else in, I would ask the manufacturer.
For what it's worth, your better off that your unit was designed for 12 AT7. There is less pressure on NOS versions than for 12 AX7. Prices for superb 12 AX7 is getting ridiculous.
