@cdc I did raise the question with Weng as you suggested, and he quickly replied as follows, which is enclosed here for your information and in case others might be interested.
That’s a great technical question. The Harmony DAC was designed with a minimalist signal path in mind, aimed at preserving the purity and character of the R-2R topology. We chose not to include an additional output buffer stage to maintain this simplicity, as well as to cater to purist systems where the slightly higher output impedance isn’t an issue due to proper preamp matching.
On the other hand, the µDAC includes a buffered output for better compatibility with a wider range of downstream gear, especially in more compact or less controlled setups. It’s more forgiving and flexible, which suits its role in smaller systems or modern integrated chains.
We are exploring the usage of the output stage on the Harmony DAC for the next generation.
I think that’s a sensible approach, although I do see the compromise involved. Some preamps, like the one I have, have a relatively low input impedance (around 10k ohms). Perhaps a switch that bypasses the output stage could do the trick—similar to the innovative design decoupling the internal clock used in both the Harmony DAC and uDAC, which includes a switch for selecting either the I²S clock or the internal clock. Some audiophiles do find that it makes a difference.