Consider not only the possiblity of a bad tube, or a bad tube pin holder, but a bad solder joint. If you are careful (very careful, high voltage) you can take the bottom off and have the amp laying on its side and running and connected to your speakers, and carefully wiggle (without touching anything else) the wires, particularly those going to the tube pin holders to try and find the bad solder joint.
Or for a shotgun approach, with amp off and disconnected, you can take a soldering iron to all the tube pin connections or others and see if that cures the problem.
Or for a shotgun approach, with amp off and disconnected, you can take a soldering iron to all the tube pin connections or others and see if that cures the problem.