5AR4/GZ34/5U4/5Y3 SS conversion

You know what your doing! I meant in the circuit though rather than in what the tubes can handle.

I have a thermsistor in the circuit, in fact two in parallel to slow it up even more. The Russian military EL84s in there will stand 315V and more with no problem, I have another amp that's been running them at 350V with no problem. Some commercially available amps run them way above 300V.

By doing this you've increased the voltage and removed the soft start effectively hitting the tubes with higher HT than designed before they've heated up. This can harm the tubes.

I did the opposite--albeit on a preamp. Added a massive tube regulated power supply to replace diodes!
See photo here

Well it certainly looks different :^)
https://cursorium.co.uk/images/ssr-3.jpg
https://cursorium.co.uk/images/ssr-4.jpg

The B+ has gone from 300V to 325V and from 290V to 315V at the OP valve plates, so as it is cathode biased I'm not too concerned. I'll put the tester inline with one of the OP valves and see what the plate and cathode current is to see if it's within limits. The EL84s were only doing about 10.5 Watts before.

I think you will still need some dropping resistors added to equal the voltage drop of those tube rectifiers!