This is an interesting one as a number of insulation failure modes exist for ageing valves. Often poor insulation may be caused due to cathode emissive material or metallic deposits which have evaporated from the electrode cage being deposited on various parts of the valves interior. Deposits can build up on mica separators or the glass pinch where the connection pins/leads enter the envelope and even sometimes the inner envelope glass - remember those manky EL84 which had been run hard and hot and built up those yukky black deposits on the glass.........?
In manufacture, the mica separators are coated with a solution of Magnesium Oxide to iimit potential leakage path length and provide a repellant mono layer to prevent surface deposition, however, the user can take measures to prevent deposition by always running their valves within recommended maxmal limits such that they do not get overrun and overheated - remember those manky EL84! Rectifier and output valves by nature of use have high anode current loadings and consequently can run very hot, however, manufacturers had designed in a number of mechanisms to combat excess heat - did you admire those sexy black anode coatings in your rectifiers ... boutique or not, alas NOT because although it looks pretty, it exists to dissipate heat more effectively - what about that PX4 with a fin mounted orthogonally against the anode ... not just there to look nice and provide a certain sound but yes, you've guessed it, it's there to help dissipate heat more effectively