[Mondial]

I would not be too concerned about replacing the 4 meg resistors. There only purpose is to act as bleeders for the tone control caps (.01 uF and 3000pF) and their value is not critical. They prevent any leakage from building up a charge on the caps and causing a pop noise when the tone control is rotated which grounds the cap in that switch position.

Do you have a tube shield on the 77 first audio tube? It is right next to one of the output tubes and the grid cap lead is very sensitive to stray signal pickup. The grid cap lead should be inside the tube shield. Had the same type of squealing problem with a model 610 where the 75 audio stage is right next to the 42 output tube. Putting a shield on the tube with the grid lead inside cured the problem.