Ahh, the old "whack it against something and see if it works" technique. I gave it a shot, free tube that is broken at this point anyway, resulted in a piece of glass from the interior of the tube (where the wires are sealed into the glass) breaking off on the inside, HA! Must have been splintered already.
So, the arcing was worst when I first turned the tube on and improved with subsequent attempts. After several, the arcing/popping stopped, but there were would be a glow that seemed to move up and down the heater filament/grid on both triodes 10s after turning on, then would stop.
I had to work over the weekend, so to test your small leak theory, just popped the tube back and did a couple on off cycles, letting it run for 10-15 minutes in between. Absolutely nothing happened on the first turn on, then seeing blue sparks on both triodes on the second and third cycles, so it doesn't seem to be improving. Again, its only when the amp is first turned on, then the tube settles down and works fine. If I wasn't concerned that one of these arcs wasn't going to harm the amp, I would say it it is still usable.
One more tidbit - the pops with the blue sparks/arcs are audible through my phones with the pot all the way down. Found this out by accident - first time I turned it on today, nothing happened, no arcing, sparking glowing. I got hopeful, plugged in some headphones, turned the pot down and turned the amp off and on again, this time a large spark and an ear splitting pop in the right channel, scared the bejesus out of me!