I moved to the Closed X headphones from HD6XX's, and have a similar reaction. I really liked the HD6XX, but I now "crave" a return to the detail, neutrality and clean bass of the Closed X when I put the HD6XX's on. Percussion and "slam" on the HD6XX's is more substantial, but also seems less focused and subjectively more like distortion to me: I'm mixed on that. I like to play the Closed X's just loud enough to bring out the percussion and slam elements; and when I do, they sound wonderful. At lower volumes, they can sound a little thin: recessed drums, etc. But that's my only qualification and the Closed X's sound better to me in all other respects. They are also easy to EQ: when I change the EQ, it comes through clearly and predictably.Beyerdynamic DT990pro, 770pro, AKG K712, HiFiMan HE400, HE560, Sundara, and few other HiFiMan's, Sennheiser 600, 650, 800.
(and many IEM's, but comparison's with IEM's, I think, is incorrect).
AKG K712, for me, also is very comfortable, but AeoN Closed X sound is better - more accuracy, lower distortions and more speed.
I appreciate Andrew or "Resolve" at Headphones.com and his thoughtful comments, even when I don't agree (which I sometimes don't). He made a comment reviewing planar headphones that I can paraphrase here, because it kind of applies to the Closed X's. He said that dynamic headphones are more likely to sound natural if your reference point in your history of music-listening is music that has been amplified electronically. That is, you are used to richer vocals from close-miking, and the additional harmonics and other distortion associated with music played back through speaker boxes and small, resonant rooms, and rather thick-waisted thwacks of the drumsticks. But if your reference point is for live music at its point of origin, you may prefer a sound with less distortion, less euphonic coloration, etc., and closed-back planars in particular have to be engineered to damp distortion and resonances of the closed-back form-factor, so that they may tend to sound more like the naked music signal. One person's neutral and uncolored sound may be another's cool and recessed sound. Now, I may be overstating or interpreting Resolve's points a bit, but it's something to think about as you listen to these phones.
Anyway, I can imagine some people wouldn't like the Closed X's, but I learned something from them: I do crave that highly-resolved, neutral, subjectively-distortion-free sound that rises naturally and coherently from deep bass to airy treble..