I'm a bit confused as to how you'd be using your headphone out to connect to the Asgard 3?
The output stage from an external DAC may have a different (probably higher) gain compared to your computer's built-in audio or soundcard, or even compared to the internal DAC card in your Asgard 3 if you have one. That would increase your perceived loudness at the same volume knob position.
In theory, turning down the OS volume may reduce resolution, but it's easy enough to try it and see if you actually hear a difference in sound quality. It might not make a noticeable difference because you are currently using the low quality DAC and headphone output of the motherboard. Both will be bypassed when you use an external DAC.
Definitely not. I have had 7 different Schiit mid-sized units with transformers built in (115V) and have never had one that hummed. I would first rule out the power cable/mains block by plugging the Asgard 3 directly into the wall using the basic supplied cable. If you could test it in another house, it would be even better. If it still keeps making the humming sound and it is loud enough to bother you, I would then simply contact their support and tell them about it.
I've never heard that humming in my A3 with MB card. I've used the power cable that came with the A3 adding an adapter that doesn't have the ground connector, and other power cable from my office with the 3 connectors and neither of them produced that noise.
