[MrFixr55]

I wouldn't worry about any of the secondaries wired to chassis ground, I am concerned about the primary of the power transformer becoming grounded. The very nature of the EMI suppression caps will cause leakage. I dunno if enough to cause a spark.

Re the loss of voltage at C31, which end of R22? The end going to load or the end going to the 6X5 cathode?

Something is loading the heck out of the power supply. Try this:

Pull ALL of the tubes except the6X5 and remeasure the power supply at both ends of R22. The readings should be quite even.

Power down, reinstall each tube one at a time until the problem reappears.
Replace the tube that loaded the PS down with a new one. If the issue recurs, remove a different tube and retest to see if the issue is a specific stage bad or total load being an issue. Check for something loading a screen or plate to ground.