Why do you suspect the bright cap? I would only think that if the issue didn’t manifest with the bright off; is that the case?
My guess is that it’s due to a bad ecap for the preamp HT node, or a bad 0V return connection to the chassis.
The V4 cathode bypass ecap may also be bad.
These things create positive feedback loops that can cause free running oscillation, given suitable control settings.
What effect does the pullboost have on the issue? How about setting the master vol very low?