You cannot change the tube type without checking and usually adjusting so that the plate voltage is correct. Adjusting means changing the bias or the plate current; changing the plate current requires corresponding changes in the regulator supply current. 6N1P for instance would need a reduction in plate current to less than half the value used by the 6CG7 and 12AU7. You would also need to adjust the heater power supply resistor if you use tubes with different heater current draw.
Tubes that do not need such adjustments are mostly those with nearly identical mu and plate resistance. Occasionally there will be a tube with a different combination of plate resistance and mu which happens to work - from the numbers, it looks like a 6DJ8 might work, but I've never tried it.
Due to the low plate voltage used (around 75v) you want a tube with a mu less than 30. A 6DJ8 or 6N1P with mu=33 is pushing it. (By the way, the 6N1P is NOT a substitute for the 6DJ8/6922/etc. in spite of popular myth!)
The 6CS7 looks interesting; section 1 is identical to a 12AU7 but section 2 could run at 8mA for more powerful drive. Unfortunately the socket would have to be rewired significantly, along with the regulator currents.
