450, 500, even 600 seems safe - and a lot of amps run around 500 with 6L6’s or EL34’s. But up around 700 there do seem to be more problems reported with ceramic octal sockets when they get dirty. With compactrons they often recommend pulling unused pins next to the plate OUT.
The first rule of Gammatron is that we don't talk about Gammatron. The second rule of Gammatron is that we don't talk about Gammatron.
https://www.itishifi.com/hifi/gammatron-257b-4e27
Let's keep it to ourselves,
Chris