Pity you don't have the HE6's to compare. Would be interesting to see if the extra power can compensate for the higher quality and balanced design of the M9. Kingwa told me it wouldn't but would be nice to have that confirmed.
I do prefer a little bit of brightness to a dull or dark sound. However I still do not consider my HD800's bright mainly because the treble is very smooth and non fatiguing and there is plenty of deep tight bass and quite full mid-range. So the HD800's don't come across as bright. However if you do have issues with fatigue then yes warmer copper would be better.
Brighter signature than the Wolfson or PCM1704 maybe but Audio-gd do not do 'bright'. Also if the M9 is anything like my M6 its treble, although very extended, is also very smooth. I also use silver cables (Digital, interconnect and headphone) and its still not bright. All IMO of course.