DC Offset Blocker/Killer - where to buy in the USA

Toroidal transformers has no airgap like old E I laminations or c-cores which means they have a very small stray field.
A gap will minimize the DC saturation, so yes DC is bad but how much before the toroid starts humming from saturation. As the transformer is very heavy it is possible it may have shifted in transit.
Is it epoxy potted or mounted on a rubber disc?
Make sure nothing is connected to the center bolt as this could make a secondary turn on the toroid, inducing a load or a voltage. Been there...
 

All McIntosh transformers are potted with a tar like substance.

This experience is saddening.  I have direct experience in Residential Electrical as well as solid state electrical design and build.  Do not buy extraneous equipment to solve this issue if you live in a house.  I know the Mac is heavy but are you able to take it to a friends house and reproduce the humming?  If yes, the Mac is bad and keep returning until good or a different model if stuck on Mac (oh no!).  This isolated circuit does nothing for 'this' issue and is a waste because you're sharing a neutral eventually at the panel...the intermediate copper ground poles is a scary idea and grounding does not solve equipment or wiring defects.  It is for personal and equipment safety that we 'ground'.  A neutral is already a grounded conductor as its called and all that is bonded with the code required double ground requirements (like the buried copper rods) at the panel as well.

I think you have a bad/shared neutral somewhere in your house (or severely unbalanced neutral).  Though fairly new, it doesn't determine the exactness of the junior electricians doing the house wiring.  It only takes one miswiring or modification to cause this,especially with 240V coming to your panel, which is then split into separate 120V sharing the same neutral at the panel, or branch circuits with a shared neutral that shouldn't be shared.  The Mac should not be affected by this however and thus it is bad.  The very nature of what it does with the incoming AC should have filtered this out.  I'm surprised the electrician can't identify the source.  Otherwise, very remotely, its a bad neighbor introducing issues from poorly implemented solar grid tie, etc. (you share power lines) but the 'electric company' can verify that as a last resort (that you have zero DC offset at the panel...where their responsibility ends).

I have never seen 'buzz' from good equipment...but there's alot I haven't seen as well.

This guy has a warrantee he should just return the component to the manufacturer for repair and while it is there hire a trained, competent, licensed electrician to correct the improper, faulty, and dangerous connections previously constructed by the prior contractor also he should name hear the dealer that sold him this component why hasn't he already mentioned that?

All the emphasis is on McIntosh's famous output transformers.  Little mention of the IE power transformer.  Search Audiogon for McIntosh + hum.  There are threads going back years.