McIntosh MA5200 hum

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.

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.