I have both, but I must say the difference is staggering between them. The Aeon 2 Noirs don't sound bad, but there's something lacking in their sound that always bothered me. I won't try to explain what is missing, but I essentially only kept the Aeon 2's because they were comfortable and sounded OK.
The Stealths are remarkable, and probably my favorite audio experience bar none. They really do sound like speakers in a treated room, the effect is surprisingly strong.
All this is to say: Small differences in measurement between these two headphones represent a massive difference in perceived sound.
It goes without saying that this is just my anecdotal experience, so take it with a grain of salt.
I did careful, level matched, bi-directional A/B comparisons between the Noire and the Closed X. When both were EQ'd to the Harman target, they sounded very similar, except as follows:
1. I liked the Closed X when I got it but thought the soundstage was very limited compared to what Drop claimed in their website. It was almost always somewhere between my ears, or at most, in the top of my forehead. When I compared to the Noire, the Noire had a much more consistent sound stage, most of the time not large, but realistic from 10 o'clock to 3 o'clock. With well-recorded material it could be quite deep and precise.
The increased soundstage also caused some difference in tonality between the Closed X and and the Noire. The Noire sounded more realistic with acoustic sources, more like sitting in an audience with balance between instruments. However, the mids could sound more recessed, and I later read Dan Clark alluding to this as part of the effort to create a better soundstage. The Closed X, particularly with vocals, could sound more forward in the mids, but there was less balance between instruments and less sense of being in an audience. This could be either an advantage for the Closed X (for example, when a good, female voice, close-miked, can sound attractive), or it could be a disadvantage (when there is less balance among instruments, and the soundstage is a straight line from ear to ear). But the part you said you are "missing" with the Noire might partly be the recessed mids, or the back-in-the-audience sound stage, however it occurs.
One would assume that EQing both headphones to the Oratory/Harman curve to assist comparison between the 'phones might erase the Noire's ostensible V-shaped FR and/or the sense of recessed mid's, but I still hear it. This is all psychoacoustics, so make your own comparison if you can. I find the Noire consistently better for classical and instrumental, and the closed X a little better for Rock, R&B, electronica and primarily-vocal recordings. Though on the whole, the two 'phones are more alike than different.
2. I'm not sure EQing the Noire to the Harman/Oratory curve is the best solution. The Noire (but not the Closed X), out of the box, has a bump in upper-bass frequencies between 60 and 160 Hz. (The Stealth has it, too.)
From Dan Clark's comments, there are two, related reasons for this bump. The most important is the reactions of listeners who are used to distortion-rich bass with dynamic speakers. As many have said, bass resonances are commonly heard to be increases in bass response, and Dan says newcomers to low-distortion planars may miss that level of perceived resonance and infer a lower bass response, when it in fact may be higher (not including the distortion).
He also avers that the bump in that FR region can increase the perception of "slam", because of the common percussion transients (like drum strikes) at these frequencies. And he notes that closed-back, planar phones need damping to prevent internal reflections, so that they may sound less dynamic.
Anyway, the bump is low enough in the bass so as to not bleed up into the mids, and I think it's a modest improvement for the Noire.
3. I found a difference in how percussion transients sound between the Noire and the Closed X. Especially with electronica, where there are fast but synthetic drum strikes, rim shots and the like, there is commonly a rapid attack and rapid decay ofr the transient, without resonance. I found the Closed X was more likely to sound extremely "fast" with such transients, like the snap of electricity, whereas the Noire sounded sometimes a little more like a hand-clap, or like a cross between a snap and a handclap (depending on the source material).
On the other hand, with acoustic (not synthetic) percussion transients, the Noire sounded better to me in picking up the acoustic decays of such transients, like the spreading of the drum strike across the drum-head, or the resonance of wood-on-wood effects, or the metal/wood timbre of a rimshot.
I heard these differences on enough selections to call them real, but it was more dependent on the source material; with some selections the differences were less obvious.
i said in another thread that I thought that people just listening to music might not notice much difference between the Noire and much less expensive Closed X, because their virtues are mostly shared virtues.
However, the time I put into all the A/B comparisons sensitized me to become a bit more averse to the "between-the-ears" aspect of the Cloased X, and more appreciative of the balance and soundstage of the Noire. But I still use both headphones, changing up sometimes based on what I'm doing or what kind of music I'm listening to.
