If you can’t figure out where the current in the spark went to then double and triple check under the board and all around for any stray wires or tiny solder balls or anything that could cause something like that. That’s probably not something you want happening very often. As I understand it, the amp was on for a while before the fuse blew. Is this correct?
It sounds like your problem is the tube socket pins are too loose. If it was making contact you should be getting at least some voltages on the cathodes of those tube sections (TP5 and 7).
Keep me posted.
Good luck!
-Brach
PS make sure you clean the black burnt area on the board and IEC jack…it’s conductive and cause arcing in the future. Rubbing alcohol on a q-tip works pretty well for this.