The EL34, did in fact, suddenly fail. I say that with condfidence due to one detail:
the top had gone white.
The 'white' was caused by the chemical reaction of the air with the tube's getter and occurs immediately upon contact. Leaky tubes will glow flourescent in operation as the leaking air becomes ionized by the high voltage and will glow purple or orange. It would have been pretty hard to miss seeing. The ionization process also keeps leaking air from reacting with the getter, so it will only turn white if the leak is severe and occurs while the tube is not in use.
The tube had a flaw in its envelope or took a whack, probably the former, as EL34's are one of the most popular power tube types and I've seen more than few that tested fine but had obvious physical flaws and it stands to reason that there would be others with not-so visible flaws. If the glass composition itself was not chemically correct, the simple process of heating and cooling during operation could have broken it.
I still don't reccomend swapping the dead tube with a replacement that has not been tested in the amp. It is quite possible to damage the amp with a tube that need more bias than the "old" ones. My advice would be to make sure that you have quality tubes rather than rely on the unknown factor of field replacements. It's always possible to have a tube fail, same for transistors. (As a matter of fact, I've had far more transistors fail in use than tubes.) Both should be serviced in the shop, not on a gig
