Is the noise coming through both channels or only on one channel? If it’s only one channel, try swapping the power tubes and see if noise switches to the other channel. If it does that would likely indicate a bad tube. You can also check that all the tubes are fully seated, I once was getting a buzz after swapping in some new tubes, turned out one tube wasn’t fully seated in the socket.
Regards,
Mario

That would seem to rule out the power tubes, the easiest way to check the input and rectifier tubes is to swap in new ones one at a time and see if you’re still getting noise. One other thing you can try is disconnect all inputs into the amp. Then turn the amp on with the volume at minimum, slowly turn the volume up. If you are still getting noise, it indicates the issue is likely with the amp.

Could still be the input or rectifier tube or a loose wire inside, but you’ll have to wait till you have some new tubes to check. Oh yeah, I’d also check that your speaker cables are tightly connected.

Regards,
Mario

The inside of the power cord input on the Zen says 250v. Could this be the issue?

I called Decware and Sarah told me the unit does appear to be wired for US voltage.

I ordered some extra tubes so I'll swap them in once I get them and see if I can find the culprit.

I have an appt. with Steve in a couple days  to troubleshoot further.

Fingers crossed that it's just a tube!

Could be a problem with the power supply or grounding. It is possible that the amplifier is not properly grounded, which may result in a humming or buzzing sound. If you do not feel comfortable troubleshooting an amplifier yourself, it may be a good idea to take it to professional service for further diagnosis and repair.

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It's been several months and I realized I've never provided the solution. Turned out it was an issue with the ground wire. I shipped the unit back to Decware for repair and it was back in my hands after three weeks, good as new.