[Arran]

The problem is compounded by the fact that there is a common cathode bias supply for all four tubes. If one tube draws excessive current, the negative bias only increases by only 1/4 the amount as if there were individual bias resistors.

  That does not sound like a very good circuit design for cathode biasing, it's more typical to see a common cathode resistor and capacitor combo for each pair of tubes rather then a quartet of tubes. As for gassy tubes, I've seen this with a #224 tube I bought over fleabay, the tube tested fantastic for emission, which I immediately knew was wrong thanks to the blue glow. If the 6П14П that was red plating was also glowing blue that would explain a lot, whomever sold it to Ron probably thought it was a new testing used tube and never performed a gas test.
Regards
Arran