Left Channel Blues

If you have another set of speakers (alternate pair), hook up

those speakers and if you hear the sound you can eliminate a speaker (main pair) problem.

This is a tube amp.  You should always suspect a bad tube first when the gremlins visit.  Lots of options for swapping components.

I'd probably just swap left and right channel tubes.

If the problem moves, then it is a tube.  I'd then swap tubes back one tube at a time...until I identified the bad tube.  

Or just do some swapping out with new tubes.

Jerry

Swap the speaker cables on back of amp, so left is now right etc. If the issue now occurs on the right channel then you know it is the amp and not the speakers, of don't forget to swap cables back. Then swap the left an right valves over on the amp etc.

You just have to go through the process of finding the criminal. This does sound like an electronics issue and tubes will do this sort of thing. Next time it goes out tap the tubes lightly with a screwdriver.

I had a similar problem not long ago. No matter what I switched around the left channel was weak. I thought something in the crossover had gone south. I eventually used my phone to measure the speaker outputs, and BAM, they were exactly the same! Turned out my left hearing aid was malfunctioning.