McIntosh MC451 amp mid-hum. Anyone have that?

Have new MC451s - burning them in - but have a hum. Have done the usual diagnostics (swapping power cords, different outlets, with without conditioner). Even when the XLR is unplugged there is the "slightest" hum.

None of my solid state (including the low solid state in the MC451 or oddly, the tweeter) have this issue.

Perhaps it is a burn-in issue. Curious if any others had this?