I've completely refrained from making "review" type judgements until I've given the pair 100 hours.
You would think that for $750, both inputs would be of the highest quality. Also I've always been under the impression that optical inputs are generally a lot more clean, and that Schiit's implementation of USB was as close as you could get to that based on the literature of the Bifrost? Also I saved $150. Even at this level of gear I have, money is still tight. I just really appreciated that I could have an "end-game" rig and not have to worry if my amp/source was a factor in headphone A/B-ing.
I don't see how that has anything to do with asking if the culprit is Gungnir being bright/harsh/fatiguing, etc.
You asked about what could be bright. Not about reviewing either unit.
I've always personally felt the Mjolnir was aggressive but never fatiguing. Gungnir was quite harsh until around 80 hours or so.