That mod looks interesting. The cathodes are indeed at 30V. I think all the component values are correct. But I had to change the HT droppers, because they had been changed before, and they were fried. So I don't know what their original values were, or what voltages they should have been dropping to. I set the supply for the phase splitter to 300V, because the ECC83 is rated for 300V, but I could have set it to at least 330V because the cathodes are at 30V.
Does 300V in the datasheet mean average or peak? The anodes will go quite close to the supply when fully driven, but mostly they sit at about 160V. (with 300V supply)
This is a nice modification, but it would be easier for me to change the dropper resistors if a 380V supply is OK.
The previous stage is a common cathode amplifier followed by a cathode follower, and before that are the input stages. What voltages do you suggest for these?