If you still have the Gungnir, my story might help you to trouble shoot the humming noise. I went crazy for the humming noise when I installed Gungnir as a replacement of "Audio-gd" DAC. With Audio-Gd there was no humming, but with Gungnir I experienced humming noise with random or intermittent pattern with different loud level at different point of time within a day. I immediately made contact with Jason and as usual he pointed there might be a ground loop. But after many many hours of observation with different tricks and tips later I found it was due to poor voltage regulation or tolerance inside Gungnir. My Gungnir was 230V model, when my main voltage drops near 225V it's start to create noise with it's single ended output (XLR of course remain noise free). There is no problem of humming if voltage raise to 235 to 240 Volt, but on the lower side Gungnir is generating noise when mains is decreasing just by -5V from the rated 230V. It is totally unacceptable, I never found any international brand which failed just at 225V which manufactured with rated 230V. For example my NAD, DENON, Marantz, Audio-Gd never shown any sign of noise even at 205V. There are many countries where 220V is found to be standard Domestic Voltage, so in those countries with the present design Gungnir will fail to operate normally if some kind of extra voltage conditioning is not used. I have mailed Jason about this. He agreed to look into this matter.
I hope you report here what Jason finds out about your situation. My experience with Schiit products has been that they are of the highest quality so I am surprised to hear what you are experiencing. That said, there will always be some units that have problems no mater who produces them.