Why is my amp distorting?


I have a vintage McIntosh 240 that recently started distorting music. I've changed tubes, preamps, wires, line conditioners; no help. I've also taken it to a repair shop where it was scoped and given a clean bill of health. I've used this amp for about 4 years with no problems until now. And I know it's not some audiophile affliction because my wife also notices the strange sounds. I'm at a loss to know what to do next. Any suggestions as to the problem?
128x128mc240
Mc:
What are the strange sounds?

Have you tried dropping the amp into another system as a last attempt to isolate the problem and/or dropping another amp in yours?

This, I think, would at least would isolate the problem unless it's a negative synergy thing.

Really hard w/ few facts given.

Was it extremely hot or humid when it happened?? I've had a few days where I seemed to get some overheating issues, but I think it may have been my DAC distorting...not my amp.
Was the "repair shop" familiar with Mac equipment? If not, McIntosh's website has a listing of their authorized repair facilities.
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