Last year ZMF came up with a patented damping system (ADS) that has been in the Atrium open, Auteur classic, Caldera and now the Atrium closed. The ADS gives each of these headphones an amazing sense of a 3D soundstage, allowing the mids, highs and bass extra room to breathe. So, while the HD250 is a brighter headphone with more treble energy, I do not feel that the resolution is better on the HD250.
The mids are relaxed on the AC vs HD250, which has more upper mid energy, but again the ADS give the mids on the AC extra room in the stage so they can be focused on better by the listened if needed.
All of that being said, like the HD250, the bass of the AC is the star of the show. When I'm using the HD250 I rarely listen to midrange focused music genres. I have the K240 sextetts for mids as well as other ZMF headphones that are more mid-centric. Similarly I don't use the sextetts for bass intensive music either.
In my headphone collection I have go to headphones for specific genres:
K240 sextetts, Atrium open, Auteur for rock, metal, blues, americana
Verite open,Verite closed, K701 for extra staging
Caldera for transparency, speed and accuracy
HD250, Atrium closed for bass centric music like EDM, trance, reggae
I feel that except for the K701 and K240 sextetts, all of my other headphones do a pretty good job of being all-arounders, including the HD250 and the AC.
Don't get me wrong, the HD250 remains one of my favorite headphones ever, but its role in my collection as the Top Dog for bass, it has been replaced by the Atrium closed.
If you're inclined, I discuss the HD250 in my review of the Atrium closed.