First thing to do is to switch the outputs between the amps (with the amps off, of course) and see if the problem stays with the amp or with the outputs. That would eliminate that possibility.
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Probably no reason to ship to Italy unless it's under warranty. Any good tube technician should be able to fix the amp. Usually (maybe always) output transformers are one per channel so that's unlikely. Which is a good thing because they are usually very $$. OTOH, sounds to me very much like it's a power problem, perhaps the power transformer?? Any possibility of the pre-amp being involved could be eliminated by running the 845 as an integrated.
Definitely swap amps, as you’ve decided. See if it follows. I can’t see how it could possibly be the ICs. And I agree, no need to send it back to the manufacturer. See if you can get schematics and specs from the manufacturer to aid whoever fixes it for you. I’ve used deltronics in Chicago to fix a ground loop on my tube pre and they seemed knowledgeable, honest and professional. http://www.deltronics.com/
Give them a call or email and see what they say. If they can’t work on it, they’ll tell you. If they can, they’ll do it right. I had a tube go out that they installed and the owner shipped out a replacement that same day. Good people.