Does anyone know what the DAC on the HDX is? It is my understanding that uses a common off-the-shelf cd mechanism. Could anybody comment on the sound quality vs a CD5si ? I would mainly be interested in using it for cd playback. I have the Ndac, but prefer not to plug it in directly to that. As I appreciate the different sound I get when using a CD player.

The HDX has a fairly decent DAC, similar in sound quality to the original NDX. Running it with an SPDIF connection into your NDAC would be much better, though.
If you don’t already own the HDX, be aware that it is not a simple thing to set up by modern standards, and that the SSD version only has storage for the OS and database. Music files need to be stored on a NAS.

In my experience, when playing ripped files, the internal DAC on the HDX is good, but with an nDAC attached, it is much better–a super combination. On the other hand, if you use the HDX to play CDs, not rips, it sounds mediocre, no matter what–definitely to be avoided.