How much more detailed and airy is the AC over the atticus? I have the atticus and am considering AC.
My comments refer to the AC with stock Caldera lambskin pads, and Atticus with stock Ori lambskin pads.
I find the AC noticeably more resolving of ultra fine detail vs the Atticus. The Atticus always resolved more than enough for me, but the AC can pull out finer detail and separate it for easier identification. I usually don't listen that analytically, but at times the AC forced me to as it was resolving things clearly that were a bit muddy/dull on the Atticus. But that's with very high quality audiophile recordings. With most music I listen to, that added resolution isn't really of much value. Crappy metal recordings are still crappy metal recordings. I do think the AC scales a bit more as well. My most resolving amp is my Stratus, and all my headphones seem clearer on it. But the AC definitely benefits more than the Atticus. That being said, the Atticus is amazing on the Stratus.
Airy is a tough one, I would not call either airy with stock pads. I think the Atticus might actually have just a hair more treble presence vs the AC, but it's not as smooth. But it's so close I'm not sure on that. Going back and forth with them, the only thing that really sticks out to me definitively is how much smoother the AC driver is up top. I wouldn't call the Atticus harsh in a vacuum, but in direct comparison IMO it's treble has a bit rougher of an edge, leading to just a bit more discomfort at higher volumes or longer sessions. But we're not talking a huge difference, and again, only in direct comparison. The Atticus is still IMO one of the least fatiguing and easiest listening sets ever.
What sets the AC apart from the Atticus IMO though is pad swapping. Honestly the only two sets of pads I like on the Atticus are the stock lamb Ori pads, and Eikon lamb pads. The suede Ori pads are decent as well, I just like how lamb sounds more than suede in general. But I have five pads I like quite a bit on the AC, and there are more to try. The stock Caldera pads (suede and lamb) are great, as are the BE2 lamb, Auteur suede, and Universe lamb. And the sound changes noticeably on each, but not in a bad way. The Atticus seems to be at it's best with the stock pads, and doesn't take to alternative pads as well (just my opinion!). But the AC transforms with pad swaps.
With the stock lamb pads the AC has thunderous bass, a neutral midrange, and soft treble but with good detail. Very Atticus like, but the key differences being the Atticus' midrange is more forward, and it's treble a bit more gravely sounding. Go to the stock suede pads and the AC becomes very balanced and linear. Very Auteur-like, with more forward mids, much more treble presence, and less boisterous bass. The Auteur solid suede split the stock pads nicely, displaying a nice blend of both pads, leaning more towards warmth, without giving up the bite and presence in the upper mids and treble. The BE2 cool things a bit but oddly seem to bring out more subbass vs the Auetur suede. I'd say it also sits in the middle of the two stock pads, with an quirkier but still enjoyable blend of the stock characteristics. The Bokeh protein pads are very similar to the stock lamb pads but give you a bit more bite up top, and mid bass punch. It's a bit more exciting and snappy, if that makes sense, but still sticks pretty close to the stock lamb sound overall. It's like hitting a loud button with them on, but without actually getting louder. Everything just seems a bit more aggressive and immediate.
I love the Atticus. It's been the constant in my lineup ever since I got one many years ago. So many have come and gone, and it remains. The AC has pulled me away from it. The AC is different, but for me personally, better. It does all the things the Atticus does so well, but better. I've been making an effort to even out my work rotation, where i do the bulk of my listening, giving the Atticus a more equal footing with the AC and Bokeh. But as much as i love it, i still lean towards the newer sets. That's not to say I don't love listening to my Atticus, i absolutely do. I just love the others just a bit more. Despite that though I've decided not to sell the Atticus. I was leaning towards it, but i just can't.
As others have mentioned, the Bokeh is a blast too. It's giving the AC a real fight for my top spot. I swear every day I try to rank them that ranking changes. One day it's Bokeh, the next it's AC. I love the Bokeh. It's not as resolving or polished/refined as the AC, but IMO it's just a bit more fun. With rock/metal the Bokeh is my drug of choice. But when more audiophile type music is called upon, the AC's refinement wins the day. Given how much cheaper the Bokeh is, IMO try grabbing a used one first and see how you like it. It might surprise you.