I had an intriguing idea to implement on the DAC board APx55_like(Actually, Amir gave me some block-diagram about how Apx555 works) THD analyzer to let users calibrate their DAC any time they wish. Furthermore, if a user will stream to the DAC as a reference signal, not clear sine but the mix of sines, for example, recorded tube preamp distorted sine, then THD analyzer will adjust ES9038 THD to minimize the difference vs such reference signal. Practically, it will look like that: you supply clear sine to the DAC and in the iOS/Android App turn On the calibration process. After a minute the App shows you THD+N result, calibration is finished. If you'll supply distorted sine as a reference, after such calibration you'll get similar THD distribution as had the reference you used(for example some sweet tube amp).
However, that idea induced near to the zero interest in diyaudio.com, as I remember.