Long story short, sound wise, Arya has better technicality than Auteur, whilst Auteur has more enjoyable frequency response and musicality. Other factors, such as build quality, are quite self-evidence.
Long story: Arya has much better soundstage, horizontally and vertically, and separation. Detail retrieval is also slightly better on the Arya compared to Auteur. For the Arya, it’s like you are in a larger hall where each instrument has better space to each other, and the transitions between instruments are more natural. For Auteur, it definitely feels a bit more blended. However, Auteur has better slam and punch, and a bit more reverby. In comparison, Arya is faster and a tad dry, you can also call it better control, it’s a preference thing.
In frequency response, the bass of the Arya goes much deeper and has better texture compared to Auteur. it’s very satisfying. The mid range is where Auteur shines. Auteur has the mid range similar to the HD650’s, but more detailed and clearer. Arya’s mid range is a bit far away and not as engaging. Arya is also brighter, as the “s” sound is more piercing. Auteur is so damn neutral that requires 0 eq from me. Overall, I would listen to orchestral(not symphonies) and instrumental music(Jazz. bop) with the Arya, and symphonies, OST, vocals with Auteur.