The Philips data sheet for ECC88 gives 90v for its typical operating condition where Gm is spec'd It gives 130v as the limiting value. (550v cold).
It also gives 130v as the max DC rating for heater to cathode voltage.
As this valve is intended for cascode operation, it seems to be being specified in cascode terms and the heater-cathode insulation may be the limiting factor and the cause of the 90v/130v rating. The higher voltage with a 'cold' stipulation reeks of heater-cathode insulation.
Then, a few pages in, we find the anode characteristic curves.... and they go all the way to 300V so much for that 90/130V rating.
So long as the ECC88 isn't in a cascode circuit, it looks like a rather good improvement on the ECC85.
Sometimes toob rolling can be the answer!
David