Mjolnir and Audezes are indeed a super sweet pairing. I was tempted to sell my HE-500 in lieu of some Audeze with that Schiit, but I think Audeze's are overpriced for their performance. Hifiman and Mjolnir is unfortunately not such a sweet pairing. I think you can use a SE DAC with the Mjolnir?
Yup, definitely this. Spent a lot of time playing with higher end amplifiers and found the emotiva easily beats "headphone" amplifiers. Save your $$$ and drop it on something else.
Using a single ended DAC with the MJ is possible, just not recommended. As for it's pairing with the he-500, I think you need a DAC like the audio-gd, a brighter DAC would likely not pair as well.