Hmm...
I think I've answered this for BOTH XDA and Head-Fi...
Listening to tracks that are prone to distortion on the M8 on both the N9005 and iPhone 5S (which really do sound quite poor compared to the M8 in terms of dynamics / power!) the same distortions are apparent there too, albeit a lot quieter (due to less volume / headroom) - so, I'm now thinking it could just be that the M8's headphone out is 'that good' that it highlights poorly encoded iTunes+ / self ripped FLAC / .WV files...
I will cross post this to both forums, so that others who hear these distortions can try this out for themselves - to listen very carefully and see if they can hear same anomalies on other players / phones...
So, might just be a case of too much of a good thing (quality / power) is actually a bad thing (and could've obviously affected the M7 too, seeing that HTC implemented the same amp design there too), which means I was deluding myself yesterday saying changing regional settings seemed to lessen the effect - Placebo huh...
Bright side though, other than picking up on poor quality rips / masters, this cements my thoughts that the M8 and Sennheiser IE800 combo is pretty darned great, and highlights that the 5S and N9005 are pretty humdrum sounding
Edit:
Simma, I will say that I DO have volume control issues, that it doesn't go up / down smoothly (sounds very 'stepped'), which can exaggerate the sound of distortion if you catch it at the wrong time!!