I have an Onyx with the Level 3 DACs and have also heard the Yggdrasil OG pretty frequently. I would say they are pretty close (although admittedly I think a lot of DACs are pretty close unless given a long listen). Both have that “something about them” that is non-fatiguing and relaxing - with the Onyx it strikes me as kind of a smooth depth to the sound, while with the Yggdrasil its more incisive and expressive. I like them both more than some other DACs I’ve had from Mytek, LUMIN, etc.
I’ve only used my Onyx with the Ambre streamer, both in the USB and I2s inputs, with the latter being better.
I’m planning to stop by the Schiit store later this week on the way to LA to try and hear the MIB if they have it. Who knows, I may leave with one and then can do some more comparisons when I get home.