Tonight (24 hours later) I powered the amps up and the same tube flashed again. This time I powered down and swapped the positions of the two output tubes. Powered up and sweet music again.
So far, after power cycling the amps run fine. I guess I get to run one trial between 24 hour cooling off periods. What tube behaviors do you associate with pentode failure? I suppose internal/visible arcing could be due to B+ overvoltage or bias error or a short circuit in an OPT. I wouldn't expect those to go away by power cycling--but anything is possible. I haven't taken the amps to the bench yet. I'll do that over the weekend.