The Supratech design IS the problem.
from EML:
SUPRATECH Grange DHT Linestage
Oct 2022.
The heater voltage of 300B in this amplifier is chosen 4.9V instead of 5V, by the designer. According to Mike from Supratech we are no tube specialists at all, and 4.9V is no problem.
Well I said, it is as follows: If you exceed a critical parameter, guarantee voids. Everybody knows that. But in the same way, if you choose maximum deviation of a life time determining parameter, we still guarantee the minimum lifetime of the tube. That is just basics.
With that particular amplifier, if you go "somewhere below" normal heater voltage, but within specifications, you get "somewhat" less lifetime, within specifications.