"** The load impedance is doubled, since the max tube current cannot increase.

Double voltage into double load = double the power. "

I think you got this ****ed up somehow. When the load impedance is halved then the power is doubled. I'll put it down as a typing error unless you argue the point.

What's more, double the voltage into the same load if four times the power, not two.

"Having the screens glowing bright is the biggest killer of power tubes."

Perhaps you are older and thus have more knowledge then I on some of this. From what I understood from the old days, tubes (valves) generally break down due to the cathode nit being able to put out current, and that is where all the current comes from.

However of course there can be other failure modes. Are you saying that some sort of arc or leakage is damaging the valve ? If so I would like your explanation of this failure mode.