First thing I'd do is check the socket that flashed and see if there's any debris or such that could short the tube. A blast of canned air is helpful.
Next step, make sure all the pin fingers are right 'n tight. Not uncommon for those to spread some when swapping tubes, especially if you rock the tubes in or out. (Don't do that.) Insert a small flat bladed jeweler's screwdriver between the socket body and each finger and adjust them with a bit of a twist. Not a bad idea to do that on all the power sockets every now and then in any case.
I'd also take a real close look at the socket that flashed, looking for cold solder connections or shorts that may only show up when the amp is cold. Heat expands metal so that the amp works "properly", but go open again as the amp cools - shrinkage, doncha know. Use a magnifying glass and also make sure the solder is all shiny with a good spread - no blobs allowed!
That don't work, time to start poking around with an insulated stick. Power up, and try wiggling the wires to see if anything weird happens - yet another indication of a weak or intermittent connection. If you find ONE, they're all suspect, and I'd consider repointing the entire chassis.
Oh. Goes without saying - always be ready to cut the power at the first sign of trouble when experimenting.