would agree with atmasphere. That first tube (12ax7) will likely have the largest impact on sonics.
Another option is the 12at7 which will lower gain a bit. but to keep everything in balance, you should change out the 100k plate resistor for a 33k, and the 1k cathode resistor for about 500 ohm resistor. This will allow the plate voltage to come back up to speck for the at7.
You can try the at7 without doing anything, but the plate voltage will drop all the way down to 50-60 volts or so, without the resistor change.
On any of the PL amps, upgrading the coupling caps, helps tremendously.
best
Another option is the 12at7 which will lower gain a bit. but to keep everything in balance, you should change out the 100k plate resistor for a 33k, and the 1k cathode resistor for about 500 ohm resistor. This will allow the plate voltage to come back up to speck for the at7.
You can try the at7 without doing anything, but the plate voltage will drop all the way down to 50-60 volts or so, without the resistor change.
On any of the PL amps, upgrading the coupling caps, helps tremendously.
best