Is this bad?

I'm thinking I'll rotate a tube from a different socket, and test it out just long enough to see if it follows the tube.

Apparently the amp needs to be biased by a technician. I could handle it with proper instructions.

Thanks for the info!

Here is a possibility, assuming that the guy you bought this from was honest about biasing before sale. I'm sure you thought about it...If he did it, he knows how, can tell you how, can tell you the optimum bias point, you can get a meter and check it out. Not to be grim, but I wouldn't be surprised if he doesn't have this info. Quite a few Chinese amps have made it to our shores w/out manuals, bias instructions, etc, making maintenance difficult for non techies. Good luck.

he had it modified for different tubes, replaced the caps and resistors, and updated the wiring. Aparently he had his tech that did the mods bias the amp prior to sale. So at worst I can get the info from him. The bias adjustments are supposedly not "user accessible" but I think I can handle it with clear enough instructions.

Was the amp shipped with the tubes in place? If not were they labelled? If the answer to both is no, then whether or not is was recently biased is irrelevant, it will need to be re-biased.

The tubes were removed, well labeled and shipped in a separate box. The amp was labeled as far as what tube went where.

I've been listening to it now for 3 hours straight, and the problem has not recurred.