How to see if there's DC at my AC outlets?

Did you try going around your house and turning everything off to see if it stops? If you are in an apartment it could be your neighbors stuff.

Does the variac hum with nothing plugged into it? If it only hums with something plugged into it then something might be loose in the Variac, but I still think noise on the line is the most likely culprit.

What's with the Variac? In my experience, all Variacs buzz, but some buzz more than others.

El, I have a Variac that has never buzzed that I noticed, but it is rated at 15 amps and I probably never got close to that limit.

Mark, now that I think about it, why are you using a Variac?

It comes as part of the Joule VZN-100s amps. There are the two monoblocks, and they are connected to Joule's variac. It supplies controlled AC voltage to the monoblocks. I suppose this way your amps always get the proper voltage that you dial in (64 to 65 volts AC), and you're not constrained by the higher and lower voltage supplied by the power company.

The outlets in my Clear Image T4 filter have 2 MOhm resistors at the the hot & neutral of the outlets. A friend told me they're for filtering DC out...