Could be a lotta, lotta things.
When you repair amps, if you take in a broken amp, there is a virtually certain possibility that it worked at one time. Those of us who have worked on amps, we have seen many Super Reverbs or Twin Reverbs or Deluxe Reverbs, all of which have many many things in common.
I used to work at a factory where electronic stuff was made. Sometimes 47 consecutive units came off the assembly line and failed initial testing. And sometimes, Consuela put that same diode in backwards 47 times. After 3 or 4, I didn't even have to look, I knew D22 was backwards. Those took 30 seconds to fix.
When you are trying to repair a kit, per your story, it NEVER worked. The difference is that you (the repairperson) have to check literally everything. Were wires soldered to the right places? Are jumpers in place? Are wires stuck through tube socket lugs or pot lugs too far and are now shorts? Are there bad solder joints? If now you throw in rotated power tubes with broken base keys, that can certainly blow stuff up and blow stuff up in ways not ordinarily encountered.
That doesn't mean it would be hard to fix it; but we who have done this can recognize certain things very fast and these things are generally not at all "hard" to deal with. But those things we recognize are for the most part 2-3-5-8 things that happen commonly in these amps that cover 95+% of all Fender faults.
I realize I'm not helping you much, but there's no great answer. It could be a hundred things, or several of a hundred things.