A big difference due to using IEM's vs speakers. It is well known that headphones/IEMs give a more accurate listening experience, combined with the isolation from environmental noise, obviously you are going to be able to hear distortion at lower levels.
But we are talking here about hearing distortion from electronics. The Wiim Pro that you described as "crap" has distortion down at around -80dB. 47dB lower than you can hear - even with IEMs. That is the Wiim Pro's distortion is 1/220th of that you are able to hear. You will not hear the difference between that, and any other dac up to the most SOTA achieving distortion levels even down at -120dB.
