Hallo,
I'm putting the Banana Boost in a tube amp, where I can find 6.3Vac and 350Vdc. So I should supply 6.3Vac to heaters and drop HT to a suitable value. Is it absolutely 12Vdc? Or could it be higher, say 60Vdc?

At 60 V HT You might probably have to adapt the resistors, although the booster should run "somehow", maybe even "better" than at 12 V. The more You rise the voltage, the closer the tube comes to its normal operation range, and smaller cathode resistors can be used.

At the O(20) V you can obtain from the input of the 7812 in Dirk's power supply kit (using a 2nd filter chain) the booster should run as designed, especially with JJ tubes which at 20 V behave similarly than old tubes at 12 V. I have this up an running in my current LoV project which i expect to run with only minor adaption also at 30 V, judging from simulations.

At the large voltages of a tube amp the banana is just two gain stages where it is a bit unclear how the current through the 470 k grid leak resistor will bias the triode. I would expect pretty much of distortion from that stage.

Thanks Bea, a really useful and sound answer