Effectively... .
Yesterday I had my first listening session with the X / 21s. The previous owner had told me that the headphones had been playing for about 25-30 hours; so let's say they were still in full running-in.
Having said that, I felt them a bit empty in the low and mid-low range, very similar to the HD800 in this frequency range; but above all I found them shrill, bordering on distortion (indeed, the sensation is just that of a distortion) in the medium-high, on female voices when they are a little screamed. So - let's say - in dynamic peaks.
With an aftermarket cable (Duelund) things on the medium-high are a bit 'improved, but certainly the problem has not been solved.
Give it time to go up to 100 hours before trying other cables and/or changing pads.
I do not equalize (first commandment of the decalogue of the perfect audiophile).
I am an LCD-X 2021 owner, and your first findings seemed to contradict most any review I've seen of those headphones. I'm glad you had come around.
But, about what's left, my thoughts:
1. They don't need burn in, this is done at the factory.
2. Even if my burn in claim is false, I had noticed no difference in their sound, over the six weeks I've owned them. Or in the Focal Clears I had before. Or in the HiFiMan Aryas.
3. Rolling headphone cables won't affect the sound, except under the most exceptional circumstances. Out of all the things you could change, that's the one that's the least likely to make any audible difference. Electrons don't care if they pass through copper or silver, in this application. Few of them are even microphonic. The whole business, is jewelry.
4. I don't think pad rolling does as much for the sound as people claim, as it's not enough to address the Audeze house sound flaws; I had put Dekoni hybrids on my LCD-3s which I had sold in the summer. Not worth prying the glued OEM pads off, even the old ones, to me. On a product that had been out for years, Audeze had just re-engineered the LCD-X to use thinner pads of a certain material; is it wise to try and undo the work of those engineers by changing the pads, at this time?
5. I have equalized every pair of speakers and headphones I had, once the technology had become available to be able to do this on a Windows PC, because despite the manufacturer's best efforts, most of them are unable to put out products that have a perfect frequency response. Their products all have flaws of some sort. Every time I change headphones, the new set comes with a new set of flaws. All Audeze headphones sound muffled to me, if I don't EQ them. I think the LCD-XC is the least muffled one they've put out, then the LCD-X 2021, then all of the others. Though the curve is flatter than it used to be, there's missing energy in the 3.5 kHz to 4 kHz region, and the response sags a bit at 2 kHz (mine is down 3 db). Audeze will also send you the unique frequency response curve for your serial number, if you ask. I had managed to come up with corrections that had sounded the same as their Reveal+ plugin.
Play "The Boy in the Bubble" or "Under African Skies", and note the impact the kick drum at the beginning of those songs has, compared to any Sennheisers. I don't see how Audeze would be able to provide that, without using large, and heavy magnets, with their 106 mm drivers. The HD800 drivers are supposed to be half that size. I've had Focal Clears, Sennheiser HD700s, Shure SRH-1540s, and HiFiMan Aryas, Audeze LCD-3s, and they were all a little bit different.