glowing red output tube, but don't think it's red-plating

Hello auidionaughts.  Had an interesting circumstance last night; output from the right monoblock began to sound slow and of lower output.  Line of sight to the tubes (both input and output) are largely blocked from the listening seating position by a ginormous power transformer and an equally sized output stage inductor, so neither me or mrs. x immediately noticed that the plates on tube #3 were glowing red...but the change in output got our attention.  I jumped up to check it out and found tube #3 as described; of course I turned the amp off. 

Inspecting the tube this morning, it shows two anomalies: silver plating on the inside of the glass opposite the ridges that hold the support rods and a very small amplitude dimple in the glass roughly centered within the two fields of the aforementioned silver plating.  All else looks normal including the silver plating on the top of the tube which looks unchanged, even though I am guessing that was the source of the silver plating now on the sides of the tube.  Could it have come from somewhere else?

During the incident, it didn't look like red-plating to me; i.e. not red at the right angle crease in the plate, rather, the whole plate was red.  Oddly, every other tube failure with these amps was fast, taking out a bias resistor and fuse and sometimes breaking the glass.  Neither of these happened with this failure and it was slow.  Was able to replace the tube and slightly adjust the bias and away we go again, so now resistor or fuse damage.  I did (stupidly) turn the bias pot down before installing the new tube ( I know doing so is good practice, but doing it removed a potentially informative data point), so can't know for sure, but I'm pretty sure that the bias was spot on where it is supposed to be (40 mA), or very close, when the failure happened.  My understanding is that red-plating is due to incorrect bias.

Any ideas what caused this failure?