I’ve had my Mjolnir 3 for over a week now. I’ve been using it exclusively with my Bifrost 2/64 DAC. Here’s a mini-review:
It is very powerful. In high gain I don’t need to go past 12 o’clock to drive my Hifiman HE6se V2 with about -8db digital pregain. In fact I can listen to them at a decent (as opposed to indecent) level in low gain with the same -8 db pregain.
It is dead silent at full volume in any mode when music is paused.
I find that I prefer feedback off. When the difference is apparent, using feedback makes music sound a bit sterile. Some of this could be due to the slightly lower volume level when using feedback. In retrospect, both of these effects are expected since negative feedback lowers distortion and overall output level.
Turning on the push-pull mode surprisingly doesn’t seem to give any additional volume, although I haven’t measured it. It does seem to give the HE6se more responsiveness. Drum hits are more impactful.
Comparing the sound to my THX AAA amp, the MJ3 sounds tube-like, even with feedback on. With feedback off, the effect is just higher. Sharp drum strikes are slightly less crisp, even in push-pull mode, but not unnaturally so. In fact, the MJ3 sounds more natural in general. A “tube amp” that can easily drive low impedance, low sensitivity planars is a good thing in my book. 
I also tried my Sennheiser HD 800S with the MJ3 and it definitely makes a good combination. Again, similar to my tube amps in both sound and soundstage. I’m not completely confident I’d know which was not a tube amp in a blind test.
I like this amp! For me it is not a huge upgrade in sound, but it is so flexible. It will easily power just about any headphone (I’ve heard the Susvara was a target during design). Clarity is on par or better than every other amp I have but the presentation is smooth and spacious.
And I love the attitude it brings. It’s the American muscle car of headphone amps. Big, heavy, powerful, and alive. 