I've been trying to figure out what happened to blow the rectifier and the fuse. I'd been playing around with bi-amping...I had shut the system down to check the cables or some such thing and probably started it back up with out much delay.
That might have done it if the GZ34 rectifier was still hot, so its "normal" time-delay wasn't happening. Most of the time we are not aware of this unless a non-delay type rectifier such as a 5U4 is being used; but turning on the AC onto hot tubes can cause a large turn-on surge. Might have been enough to push the rectifier into failure. The KT66 that arced over probably won't ever be the same, either.
That happened to me, on a Mark II. One arc was all it took to kill a Mullard EL34. It arced every time it was turned on, after that. It had seen ten years' service but was still working just fine.
It happens...that's all I can say.
Oh, I forgot to ask: you DON'T still have the original Dyna quad filter capacitor in there, do you? AKA "time bomb."
One further note: If you saw a tube arcing, it would have helped had you told us in the first post. That was a related part of the failure...it wasn't just a fuse blowing for no reason. The more information, the better.