I compared it against the DCA Corina, the Susvara, the HD800s, and my L700mk2 in the private demo. It was originally meant to the an X9K vs Corina rematch on the BHSE after hearing both on the LTA Z10e at CAF, which some people said was underdriving the Corina. I think the BHSE is indeed better for the Corina (Dan Clark uses the BHSE in-house for development), but it didn't change my preference for the X9K.
If you're interested in reading an essay, I wrote up my detailed comparisons from that demo
here. The in-depth impressions on the X9000 itself are two posts above the one linked. I took written notes during that demo, which helped a lot in collecting my thoughts on it later.
Earlier, at CAF, I compared the X9000 against the Empyrean 2 on the Z10e. The X9000 won that matchup handily. The X9K was smoother (fewer notable peaks) in the treble, had more snap and kick to the leading edge of bass (Empyrean 2 was soft-sounding in comparison), and the midrange was "sweeter" on the X9K. The Empy 2 was still a very good headphone, but I would actually pick the Empy 1 over it, especially since I now have the X9000. The Empy 1 has that bassy and "dreamy" character which is distinct from the X9000, while the Empy 2 moved towards neutrality. But all of the attributes that the Empy 2 acquired vs Empy 1 were attributes that the X9000 does better still.
I briefly compared the X9000 with the SR-007mk2 on the BHSE. I thought the X9K was more comfortable and sounded more neutral. The 007 had a weirder midrange/treble which tonally reminded me of the L700mk2. It was darker overall, but with a bit of sparkle in the upper treble. I probably should spend more time with the 007, but I tend towards brighter sound signatures, which is a better fit with the X9K.
I've heard other headphones (LCD-5, CRBN, MM-500, both Tungstens, Stealth, YH-5000se, and briefly the Expanse), but not in direct comparison vs the X9K. Other than the Yamaha, I wasn't really interested in more demoes of them. The Audezes sound kind of shouty to me, the Tungsten is good, but nothing really stood out (though it's sort of a continuation of the old Audeze house sound which I find better than the new shouty Audeze sound), and the Dan Clarks are extremely detailed and tonally even, but dynamically dead and just not engaging or interesting.