Got a set of these a couple of days ago. They are absolutely amazing. My reference closed cans were Focal Elegia. I don't love their build/comfort but really liked the the SQ. I also had a set of AFO and LOVED their build and comfort. I'm working from home a bit more these days and so have been using the closed cans through the day. I occured to me that I really should have the AFC as my comfy all-day work cans and the Elex as my open cans, not the AFO/Elegia combo. So I started looking for some AFC but a great deal came up on these locally...
The Ether CX is not quite as comfortable as the AFO, the extra weight is noticable. But the build is much nicer to me on the Ethers. But the SQ of the CX is incredible. The only thing the Elegia does better is dynamics. The CX has better sound stage, resolution and imaging. I think the CX retrieves more detail too and it does it more effortlessly. What I mean by that is the Elegia feels like it's really desperate to put detail right in your face and sacrifices timbre to achieve it. It's like the 6XX v HE4XX, it always amuses me when people say the 4XX has better detail retrieval than the 6XX, it doesn't (it's not a bad headphone though), it just gives that surfaace impression because of it's less natural timbre IMO. The CX v Elegia is a simlar comparison I think. The Elegias are great headphones. I just think the CX exposes all their flaws.
I don't find them sterile or boring at all. I'm using them without any inserts and with a Lavri solid silver cable. I'm still experimenting with the them but my feeling at this point is they don't need them. I have a schiit Loki though, so I can tweak things with that if some recordings need it. Source gear is Eitr>Mimby>A90. I think the A90 is a MUCH better match for the CX than the Elegia, so bear that in mind.