Project86's answer was related to differences pertaining to using the Concero (SD) and HD as converters. The digital transport path through the two versions should be similar, but the DAC sections are notably different, especially since the ESS9023 and ESS9018-2M differ a lot in architecture, the former being more modular and the latter requiring more additional supporting circuitry.
From my opamp rolling experiences I'm pretty sure that the AD based I/V stage in the HD has something to do with its more caffeinated sound signature. My headphone is caffeinated enough as it is, which is why I went with the standard Concero. Again, a lot of people on Head-Fi overlook system synergy for wanting the best all the time.
True - sorry if I wasn't clear enough about that.
Personally I find the HD superior in enough areas where I think most folks would prefer it. But, I can also see how the increased resolution and detail retrieval would tip some systems over the edge into "overly analytical" territory. So original Concero is still definitely worthy of a recommendation.