I've built a PSE 6S4A SET amp, with each channel using using two paralleled 6S4As (each with its own 390 ohm cathode resistor and bypass cap), driven with an SRPP driver stage, I used 5963s, the 12AT7s I started out with, had way too much gain. I used RK and RA of 1K ohm,with the RK bypassed with 100 uF / 16V cap.THis runs theSRPP stage around 4.8 mA,the 12AT7 with the same values, ran about 2.1 mA.
I used some cheap 6.6K PP OPTs as SE OPTs, they are not airgapped, but they work and sound pretty good.
If you go with 3 64As pr channel, a 4 to 5K SE OPT should work well, the three paralleled tubes will consume a totalof approx 75 mA plate current, so make sure that the OPT primary can handle the current, look for at least 90 mA primary current capability, the GXSE15-8-5K should work well in this application.
/edB