Little help please

Have a bit of an issue, and you people here have always been extremely insightful.

Sat down to listen tonight, and came across an unfamiliar issue. Sounds fine for five minutes, then fades out completely for about 20 seconds. Wait 15 seconds or so, fade back in. Wash, rinse and repeat about five times. No static. Just a slow fade out and fade in.

Using a MastersounD Compact 845 amp in direct mode through an EAR 868 pre-amp. Source is an Ayon CD5 player. I switched over to Frankenstein 300B monos, and it works fine. That should pretty much eliminate the pre-amp and CD player as the problem.  Note that both the 845 and the 300B's are both connected to the  pre-amp. They run through outputs 1 and 2. When I want to switch amps, all I have to do is change the speaker cables at the speakers themselves and turn the appropriate amp(s) on and off. Simple set up.

Sounds like a problem with the 845, correct? Could be an issue with the output channel the 845 runs off the pre-amp, but that's kind of strange to have both channels go at once. Tubes on the 845 look fine, and would also be strange to lose both channels at once with a tube issue. Don't think it's the IC's I use on the 845- that would also likely be one channel instead of both, and usually comes with a lot of static. This is just silence.

Output transformer, perhaps? Any ideas for a potential quick fix, or is this something that I should just pack up the amp and send it back to the manufacturer for a check and possible repairs?

Thanks in advance!