I assumed this thread was over, and here it is risen like a phoenix
. As an owner of both the HD6XX and the Auteur since January of 2020, I have had an extended time to compare them. While I do still really like my HD6XX's (particularly with some "corrective EQ" - see for instance Amir's recent review of the HD650), the Auteur really is for me clearly superior. As it should be given the price differential.
I'm no audiophile reviewer, so I won't try to use all the typical verbiage. Listening to the HD6XX is always a good experience, they sound fine to me. Donning the Auteur afterward, however, is definitely different. First, the Auteur just opens up a another level of listening. More "open" and a wider soundstage that results in a sound more akin to a "being there" experience.
Also the quality of the music is discernably improved. By quality, this is a combination of things. "Realism" is another term that could be used. The Auteur conveys information across the entire spectrum from bass to highs that I just don't hear with the HD 6XX. It's difficult to describe.
With the HD6XX I hear the thump of a bass guitar, but the Auteur somehow lets me hear more - like the character of the instrument and the strings are heard. Very fuzzy to try to describe as I said, but I can certainly hear the difference. Bottom line the Auteur brings an enjoyment that makes them worth the price for me. I love them.