This is similar to what I suspected, though I was planning to put the rudimentary shield to a different place. But let's start from the beginning. Today I removed the bottom plate of the amp to see if there is any obvious problem inside. I found nothing strange, so I screwed it back and continued my day. Later I wanted to check something, but I was lazy to download the photo from my camera, I rather used purr1n's picture (here). This is when I noticed that it's different to what I saw in my amp, so I downloaded my own image. This is it: Do you see the lots of differences? One very obvoius of them is that on my ZDT the small transformers are not shielded from the internals of the amp. Does it matter? Maybe, I don't know. I didn't check what's behind the PCB. May the main transformer be also not shielded? There are many other differences between my (humming) ZDT and purr1n's pre-production (silent) model, but I'm not an electrican, so I don't know which difference is meaningful, and which isn't. I also noticed on your chart that the frequency of the humming is around 60Hz, and 108Hz. Taking into consideration that you're in the USA, where the power outlet's voltage is around 110Hz, it's seems reasonable to believe that in Europe we hear 120Hz + 230Hz humming instead. Being a higher frequency, it might be more obtrusive.