Hi Harry,
I wouldn't worry about the 500+ volts. The tube just sees the difference between the plate and cathode voltages. I suppose it is possible that there might be a small fraction of a second somewhere during the turn-on process where that difference is significantly greater than the much lower value it subsequently stabilizes at, but I'd be very surprised if that could cause any problems. And I see that the tube is rated to withstand up to 450 volts, provided that the 450V does not persist for a long enough period of time, in combination with high current levels, for the resulting power dissipation to be a problem.
The 450V rating, btw, is the same as the rating of the original Western Electric tube. I can't see any reason why exceeding that number by 10% or 20% or so for a small fraction of a second, if it were to happen at all, might cause a problem.
As far as the second part of your question is concerned, given the precautions you would be taking, and the substantial margin between the 28W absolute maximum rating and the 20.5W someone measured with the same model amplifier but probably with a different type of 300B, and also the margin between the 65 ma he measured and the tube's 80 ma maximum current rating, intuitively it strikes me as a reasonable gamble. But hopefully someone with knowledge and experience that is more directly relevant than mine will chime in.
Best regards,
-- Al
