What happens to hiss when you remove your test probe from V2b-7?
Hiss might be "what you hear" from an oscillation and probe could be an interference injecting antenna.
What do you see when scoping V2b-6 and you rise/lower volume pot?
At least, anything you see on pin 7 will be much more visible on pin 6 .
Also, what do you hear?
It should match what you see on screen.
I´m not much worried at schematic indicated plate volotages, if anything I find *them* high, go figure.
What you measured sounds more usual to me.