IMO your tech is a (fuzzy little kitty cat). I barely need to take my socks off to count the parts inside one of these. "Could cost a ton of money to troubleshoot?" What a schnozzle! I have no difficulty killing EH tubes or similar "Tung-Sol" reissues. It ain't 1965 anymore. Russian tubes sure ain't RCA or Sylvania. Going around in circles. Schnozzle tech already passed on it and handed it back to the customer. I'll take a WAG: A zorched "Tung-Sol" 6V6 gave the cathode bypass cap a blast of high voltage. The cap isn't designed to endure 300- plus volts. Seems like the heroic little cathode cap re- formed when its environment returned to more or less normal. The circuit will function with the bypass cap entirely removed. It may function better with a new one in its place.