Hi all again.
Today I believe all chassis should be grounded thru a three prong ac cord as I did when upgrading my PL with the WO new boards and backplane kit.
There's a good chance that transformer can short to ground being old.
The guys told me not to do because of ground loop problems.
I feel safer having all equipment attaching to amplifier is best.
If a ground loop should occur, try trouble shooting the cause rather than having unsafe equipment.
I have no problems with hum with my system.
Wayne