OUCH! UT. Interesting though, as when I was researching alternatives to the 6L6 there seemed mixed views as to the 807's versatility? But perhaps it was indeed simply a bad tube which, as you said, can also happen to NOS tubes alas. Normally that kind of catastrophic failure seems to be down to either arcing across electrodes or too high a voltage on the anode?...
And as for silver, if anything in the system is over-sensitive to treble frequencies, then (good!) copper might be a safer bet. However,
mono-crystal OCC silver, as with Neotech's UP-OCC offerings shouldn't cause any such problems (the opposite in fact), and also still deliver good, solid, detailed bass. As always though, the only real way to be sure is
suck it and see! ...GOOD LUCK... CJ