At those voltages in an SE amp you need more like 700+ ohms. The tubes probably were dissipating way over spec, with only a 470 ohm resistor in there.Careful, I've taken out a few JJ's in SF champs, maybe it had to do with the 400v on plates + the 470ohm cathode resistor.
I have run various 6V6s at up to 450V, including JJs and 6pi6s. I've also seen JJs running happily at 470V in a friend's amp (which also had tone to die for BTW). Once there was an electrical fault in his house and he ended up with 500V on those JJs - and they continued to work quite normally.