I think you isolated your problem. Have BAT check it out.
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Do you have the old tubes? Can you re-install them and see whether the phenomenon recurs? That would establish that you have one or more defective new tubes. What do you mean when you say some of the new tubes tested "weak"? Does your tester measure transconductance? It is not necessarily the case that a tube amp and solid state amp are per se different in their tendency to blow fuses. It depends entirely upon the two different circuits, how much current they draw, and how much safety margin the designer decided to go with, when he or she specified the current rating of the fuse. One possibility: one or more of the tubes in the VK5 is losing bias voltage on its grid. As bias goes toward zero, the tube draws more and more current, which is wearing out the tube at best and could cause the tube to self destruct if allowed to continue for a long time. But I don't know how to tie that scenario in with blowing fuses in the Pass amp or drawing a lot of current from the ARC amp. Another thought, if the VK5 is capacitor coupled at its output to the amplifier, and if that capacitor has failed, THAT could send DC into the downstream amplifier, which could upset the bias of the input stage in the amplifier. I like that idea better.
I suspect the output coupling capacitors. DC at the input of any amplifier with a direct coupling input will give it conniptions. With solid state blowing a fuse would be one scenario. With a tube amp it would cause the input circuit to not be at its proper operating points and so would likely make more distortion.
Some of the older VK-P5 used coupling caps that look like oil-filled parts. If that is what is in this unit, that would be my first guess. But since you're sending this back its probably moot.
I always had a feeling that in the good old days, with the good old oil and paper capacitors that contained PCBs, now outlawed, leakage was less of a problem. I have had a personal bad experience with one or two of the modern era oil capacitors, and I would no longer ever use one in a couple in capacity especially.