I'm puzzled by What Hi-Fi's website and magazine reviews of these three DACs, having in recent weeks burned in and done extensive listening with all three via my desktop computer (associated eqpt: Emotiva airmotiv4 speakers with Supreme fuses, My Audio Cables and Morrow ICs, and MAC HC and Emotiva X power cords; no headphones).
The HRT microStreamer is very attractive for the reasons you give, but tonally off. That is, it's gots lots of the pieces of really good sound that one yearns for with every device, but singers don't resemble real singers and instruments don't sound like real instruments. That was my reaction from the first listen until the frustrated last, when I returned it to the store. I suspect the problem is that its sound is very dry, and dry is tonally off (ask musicians). I recall reading a comment somewhere in a review of an earlier HRT Dac that dry is the house sound.
The Audioquest Dragonfly certainly doesn't get all the pieces the HRT does, but tonally it resembles the real thing, real music. That was immediately obvious from first listen and hasn't changed over the course of 200 hours of burnin. In addition, I haven't found the Dragonfly inordinately forward or fatiguing on good or bad quality (or old) music files and CDs. Perhaps that has to do with associated equipment (those Supreme fuses change the Emotivas' forward perspective and other qualities a lot).
As for the Micromega MyDac, currently playing here, for the life of me I don't hear your review (nor checking around afterward have other reviewers). On MyDac's downside, contrary to your comment of it as a "fast-paced and exciting perfomance," I find it to be a bit slow and not nearly as powerful and exciting as the microStreamer. That shows very clearly when big bands strike it up. OTOH, the hard edge and degree of brightness you report, well, I don't hear it - and I'm really sensitive to brightness and fatigue (another reviewer refers to it as "a little sparkly...less of a natural barrier to the roll-off of high frequency content") Rather, this baby is musical (tonally correct), has lots of detail, along with an ease and a breadth of soundstage, plus a lightness (a bit too much of it), that easily leaves the microStreamer behind, price differences aside. There's no question that the MyDac would benefit from a better quality power cord and probably some weight on top to damp its plastic box as well.
I'm not sure if these differences in appreciation are the result of unit to unit variation, associated equipment (even the sense the power cord is plugged in - U.S. style, at least), or more profound differences in sonic preferences, perhaps cultural in part. I know that some people are captivated by sound (detail) and apt to become so engaged by it that they come see it as what real music sounds like. That was my first thought in reading your review of the microStreamer.