As a diagnostic tool only I would try a cheater plug on the amp to see if the hum disappears. If it does then it sounds like you have a ground fault issue and perhaps running a dedicated circuit to your system may help.
I've now checked out everything that audiogon offers to remedy this but still a hum from the amp (not through speakers). Very dissapointed. No choice but to return to dealer unless anyone has any other suggestions.
I will go back to listening to my YBA and never go near Macintosh again.
What a waste of time.
Almost all transformers hum a very little. Many are pretty much inaudible. Some are not. Broadly speaking, transformers in tube amps hum more than in SS (there are exceptions). Most guides will tell you that amp transformer hum is one thing, and hum through speakers another. No guides tell you how to eliminate hum when there's some hum in both (as other users here on 'gon have individually reported). If you're lucky, one of the standard remedies will work. Otherwise, just return it. It may be unfortunate synergy between your AC and the component, or the transformer in your unit may have been manufactured on a Friday afternoon. After exploring this quite extensively, I don't believe that there's a power conditioner/regenerator/isolation transformer out there that will predictably remove all hum from all transformers. Sometimes they will work, sometimes not. The fact that there's no hum at the dealer's definitely suggests your AC, but if you've worked through the checklist, save yourself the hair-tearing and return it. BTW, Ayre amps are particularly quiet.
It’s silly to conclude that one must stay away from McIntosh because of the problem you are experiencing. If you like the amp, keep it. Send it off to McIntosh with a note of your issue. They will most likely repair it for free and bill you only for the shipping. This has been my experience as a long term McIntosh owner of many amps and preamps.
I find their service and build quality second to no one.
This will work if there is actually a fixable problem with the Mc. Experience tells me that there are some transformers when fed with certain AC supplies will hum, regardless of whether the transformers are entirely within spec. Same experience also tells me that there isn't a power conditioner out there that will transform the AC enough not to produce that hum, if it's somehow inherent. So worth a try, but be prepared for a negative outcome.
Late to this dance, I know, but I have a MAC MA5200 with no hum issues. I've loved it for 3 or 4 years now. My suggestion? Call an electrician and have him or her check your outlets. Of course, you can do that yourself if you know what you're doing. Please don't electrocute yourself. When dwellings are built, electricians sometimes make mistakes wiring outlets. It's rare but has happened to me. I had 1 outlet in my house with reversed polarity. I'm no electrician but I'm thinking that might be the source of the problem.