This is very interesting issue with DAC. I do hope you are able to track down the issue and fix it.
I do have a portable DAC which I use seldom with my iPhone and Mac. It is NuForce uDAC-3. I realize it is not high end gear because portability is important to me. I can give it a try and test it with next beta (I am hoping for a Volume Boost fix) to see if I get an error.
However, I use headphones so recording with iPhone won’t work. How would you suggest to capture audio? When DAC is plugged it has to be selected under Output, and any software that will record the output (SoundFlower…) has to be selected in Output to capture audio, so that would bypass DAC.
It would be good idea to point us to areas to focus on or try DSP under certain conditions/settings and report. This way we might give you a better and more useful feedback to what is really important for you rather than reporting things that you are already working on and are aware of.
If Eric’s recording does not give you the clue and headphones/computer audio capture is not possible, then the only way to go is to try to reproduce it yourself with FLAC file (preferably from Eric) and similar setup.