I'm getting a hum...

AMC CVT 2100a valve amp, Zu Druid speakers, iDAC, Apple TV.

I disconnected all sources, removed them from amp, then powered up amp, with speakers connected and got a low volume buzzing hum.

Then connected iDAC, powered up, and got a lower volume hum with out the buzz.

Tried different wall sockets and surge protectors, made no difference.

Switched out amp for a friends, no hum with all the same sources.

I've read on other posts about upgrading powerchords. The manual says "if there is a hum make sure power is grounded via power cable", but, doesn't say if this is anything different from simply having it plugged in...

Recommendations? Should I get the internal grounding checked out?

Thanks