To me if both well implemented, it comes down to the type of conversion used, Delta Sigma or R2R Ladder
The AR 9 uses PCM1792 dac chips which are Delta Sigma, not my favourite, my fav being the R2R ladder dacs PCM1702 and the even better last made PCM1704.
The AR6 I can’t find what dac chips it uses, I looked but there is hundreds to look through, Search "Audio Reseach CD6’ and look for dac chip, dac’s, or D/A converter. If you find it let me know, as it "seems" to be a bit of a secrete.
Cheers George