I'm finding, after the initial honeymoon period has worn off, that my Aeon Closed X was lacking something. Couldn't put my finger on it for days. Kept trying different EQ corrections from Oratory1990, Robbo99999, this thread, and everything in between. Just couldn't balance it out for some reason. It just always sounded a little thin with no low end grunt when I pushed it. Even my HD-650's with a bass shelf seemed to outperform the Closed X in bass/sub-bass! Every time I tried to add to the bass shelf of the Closed X, it just didn't keep up with the adjustments. The bass (especially sub-bass) never came in at moderate volumes, and the overall sound just started to fall apart getting bright and painful.
Why was this happening? My theory was my amp (Topping DX3 Pro+). As stated earlier in the thread, the actual sensitivity rating for the Closed X is unknown until properly tested. I'm of the belief that DCA wouldn't put the work into increasing the sensitivity by that big of a margin for Drop. Minus actual testing, it's more likely somewhere in between, and I'm guessing now that it's closer to the RT. Amir's measurement of the RT works out to a sensitivity of roughly 78.6 dB/mW (13ohms), needing an amp testing well with low impedance and low sensitivity headphones.
So, I made the assumption that the Aeon Closed X is closer to an Aeon RT and started calculating some voltages with Amir's measurements. Based on the RT, a 102 dBSPL would require 1.69V, already on/beyond the edge of what the Topping DX3 Pro+ could provide at 12 ohms. Trying to play the Closed X loud with the DX3 Pro+ would be troublesome. Also 'seems' to explain why I couldn't get the positive bass shelf adjustments to sound right.
Conclusion? Minus actual testing data, I took a leap and bought the Topping A30 Pro (couldn't quite bring myself to grab the A90 Discreet). The Aeon Closed X came alive! Adjustments of the bass shelf started to work as expected, and the sub-bass came in absolutely beautifully. Everything seemed to balance out very well. More importantly, a side by side comparison (volume matched as close as possible by ear) of the DX3 Pro+ and the A30 Pro amps confirms (in my head at least) what I had initially thought. Pushing the DX3 Pro+ quickly causes problems even at relatively low listening volumes.
TLDR: The Aeon Closed X seems to require an amp testing well with low impedance AND low sensitivity headphones. IMHO, Drop's sensitivity listing is most likely wrong. A DAC/AMP combo like the Topping DX3 Pro+ (as fantastic as it is for many headphones at its price point) just didn't cut it with the Closed X. Until the DCA Aeon Closed X is properly tested, I would suggest people treat it like an Aeon RT and its associated power requirements and NOT based on the numbers from Drop. Like Amir has said, take the amp out of the equation budget allowing (at least I think I remember him saying something along these lines). With a new Topping A30 Pro amp, I'm finding the Oratory1990 EQ with a couple of dB added to the bass shelf is absolutely amazing. The Sample #2 EQ in this thread seems to recess the vocals a little too much to my ears.
YMMV. I sure hope this isn't all in my head.