Thanks for the video @miyagi1218, and for your patience.
We actually have a Merging NADAC in-house and we pulled it out this morning to make sure it’s working for us on OSX. We’re not having any issues here, so something is different in your setup versus ours. I’m sure we once we figure out what that is, we’ll be able to move forward here.
First off, have you installed the CoreAudio driver they provide?
Second, have you confirmed the DAC is tuned to the right source? It seems weird that the DAC would ever start when this was set incorrectly, but the behavior of this DAC can be a little different than what you’re expecting and sometimes you need to pick the name of the computer from the front panel. (If you need more detail here, let me know. Sadly for me, the NADAC isn’t here with me at the moment 
Finally, I think a few of @ncpl’s questions above are really good.
Screenshots of your audio settings for the NADAC (like the two shown [here] (I'm having a problem with Roon -- where do I report it?)) would be really helpful. You can take screenshots on OSX using these instructions, then just drag your replies into your response here.
I’d also like to know the answers to this:
And this:
You can do this by visiting the audio tab of Settings, typing a name in for Built In Output (as shown in @ncpl’s screengrab above), then selecting that zone from the zone selector, here:
Finally, I took care of this one:
Let me know the answers to the above, and then we can move forward and gather some more logs here if we can’t figure this out. Thanks!