Currently at about 12 hours with my Schiit stack (see avatar) and I have some quick impressions. Opinions are based on switching from an iBasso D7, which I used constantly over the last couple of years. Headphone used were my stalwart companions over many years: Denon D2000, unmodded save for J$ leather pads. I also own, and religiously use, a pair of Heir Audio 4.Ai's, but I have yet to try them with this new gear, as I use them most often during the week, as the cornerstone of a mobile rig.
Anyway, first impressions of the build quality and appearance were very good. The brushed aluminum looks gorgeous and the fit and finish is pretty good. I had to dig up the screw-on 1/4" adapter for my cans, but once I found it, setup and prep was very quick. Driver installation for async USB on the Modi was painless and quick.
So, about the music and aural impressions--the real reason you're likely reading this: "precise," "clean," and "detailed" were the first words that came to mind when I queued up the first song. My face went pale and I panicked as it initially sounded like this gorgeous new rig had neutered the D2000's signature warmth and impressive bass performance. I had never really considered the D7 to be a warm amp and DAC combo, but in hindsight, it clearly was just that. The Magni/Modi 2U stack, even after a dozen hours of usage and listening, really thrust home the "solid state" signature I had heard used to describe non-tube amps: clean, precise, revealing, detailed, and neutral. Others may disagree with this assertion, but I'm simply expressing what my ears (and brain) tell me while I listen. The unexpected treble of this combo's clean sound will still surprise me as I go back and listen to favorites and new albums alike. But then I'll hear details I've never noticed before and I get a smile across my face.
I plan to continue listening to this stack, and I expect it to continue to impress me as I get used to its sound. It does seem to be highly track/album-specific, as well as insanely revealing of quality. Some tracks will have harsh treble and be sorely lacking in bass quantity, but the details and bass tightness and precision will shine through. Additionally, FLAC (and 320kbps MP3, e.g. Spotify Premium) can certainly make a huge difference, especially when it's a true 24/96, 24/192, etc. master. In those highest-quality of masters or personal rips, many of the quirks in the sound signature that jump out at me seem to fade away and I hear my D2000's perform the best they ever have in the years I've owned them. I assume that's due to master, mixing, and artist, though.
Anyway, those are my not-so-quick impressions. Looking forward to spending dozens more hours with these handsome and precise little boxes.