Is this the sound of tubes going bad or something worse?

[Silent Sound]

Clips or it didn't happen.

No, seriously, it's hard to tell. A recording would go a long way to help identify the problem over the internet, but even that may not help unless someone has heard that exact same sound before. I've not heard a bad tube create a vibrato sound. Tremolo, maybe. In any event, the first thing I would try is to swap out the tubes for known good ones, and see if the problem goes away. If it does, then you'll know it was a tube problem. If not, take it to a tech.

[davet]

That warbly tone reminds me of a MESA amp that went south during a gig. It was the power supply.

[qvplite]

Definitely try swapping all the tubes out for known good ones and see what happens. Even with low hours, some tubes just aren't cut out for the test of time.