I was listening to my Mk IIIs the other day when I heard a "crunching" noise. I immediately turned off the amps. Yesterday, I powered them up without the cages and watched. After a minute or two, I heard the same noise and saw flashes in one of the output tubes (Electro Harmonix KT88). I swapped the tubes to the other sockets, powered up the amp, and watched while measuring the bias. There was no crunch or flashes, but once the bias hit 1.56 v, it hovered there than promptly soared over 2 v, at which point I powered down the amps. I checked the bias with another recitfier. It still rose, but not as fast. I swapped in a pair of 6550s, and now it is holding 1.56 v. Empirically it seems I had a KT88 go bad. What else should I check? Thanks!