Is this bad?

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.

Most likely your amp has bias adjustments for each output tube. Each output tube needs to be in the same socket that it was in when the tech performed the bias adjustment or you have to start all over with bias adjustments.

If the seller took the time to mark each tube for the proper socket before he shipped it, you might be able to move them around to their proper location and be up and running again. If not, and if the tubes were simply shipped independent and without any indication of which socket they go in then the seller didn't take his responsibility of selling serious.

Tubes can vary in how bright they are and still be biased properly. A tube with a filament that extends higher above the plate structure than another will glow brighter. However, if the plates are the least bit orange or red then you have a problem.

yeah, about the tubes are what is being biased. So moving the tubes from one to another sport will change what the bias in each circuit is supposed to be. Did the tubes come separated from the sockets? if so, did you get them in the SAME one they were from when the bias was set?
(I love tube stuff... but I can't stand the tube bothers! ... maybe after I retire.. tubes. Mmmmmm)