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?
One channel or two? What about the speakers, have you switched them? Do you have more than one source component? You probably have already thought of all of these, but your posting doesnt mention them, so just in case....good luck.
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.
Could be the dreaded cold solder joint.