Musicians Roadhouse

Hi folks!
I'm a french tech guy working on a JVM 2266C amp
It has a weird buzz (100hz and harmonics, i'm in europe) that occurs only when the master is at middle position (goes away at 0 or 10).

I tried all the usual things and it's still there.

i disconnected CONN5 so preamp is not connected, disconnected pin7 and pin8 so 20VAC rectification is off.
i removed C52 so 3.15VAC rectification is off.
So it's just the power amp now.
and i still got that weird hum, even higher that before! (not depending on any settings with the preamp disconnected off course)

I changed all electrolytics caps (filter caps, bias caps)

When the amp is on and warm, if i disconnect the heater the hum stays here until the KT66's get cold. So it's not heater hum.

KT66's are new, and biased. It is the dual bias version with trim1 and trim2
I noticed the hum goes away when i set trim 1 to a very low value.
I'm short of ideas here, seems theres something wrong with the bias supply but i changed the bias caps and the bias diode, same thing...

Any help ?
thanks !

Have you scoped the source of the buzz?

Have you tried replacing the Dual Master Volume pot itself?