Just got new zma with zp3 and am experiencing very audible him. After a debugging session here is the simplest setup that can reproduce the issue.

No input into zp3. Zp3 in to zma. Zma into Cornwall 4. Hum rises with volume. Zp3 and zma plugged into power quest 3 (same result with cheapo power bar). Nothing else plugged in. All cables separated as much as possible.

I ruled out sensitive speakers, as I reproduced hum to a lesser degree on my less sensitive kefs.

Zma is ruled out as I used tt into audio labs 6000a in pre amp mode, then to zma and speakers. Whisper quiet.

. Any suggestions for what I could try next?