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

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...

PS Audio makes a "humbuster". It appears to be a DC blocking device and is reasonably priced (ay least as far as can be said for an audiophile component). PS Audio claims it will quiet mechanical transformer noise. They are designed for high power loads so a power amp should be OK. Maybe a dealer would let you borrow one to see if that does it.