Marshall JMP 50w volume drops shortly after standby engages power

[Thorny]

The amp is basically a Marshall JMP Master 50w. When initially warmed up and brought off of standby it sounds fine, the nearly immediately it drops in volume and is all distorted.

No FX loop to cut out
Tubes test good in tester and subbing 12AX7s does not change things
Nothing appears loose after doing chopstick test
Electrolytic caps test good (ESR)
Voltages look fine
Cleaned pots and jacks and switches, including the standby (tests good)

Also, it gains volume briefly again when you turn the the standby in the off position until the power dissipates from the circuit. Guess I might need to dig out the scope. I wonder if the stuff I actually continue to hear is the negative feedback?

Probably something simple but I am scratching my head.

[Alex R]

Lack of ground reference at grids can do this.

[Enzo]

Bingo, that was my instant reaction.

Power down and measure resistance from every grid to ground.