Well, I have to elaborate on that. It's impressive in some ways, but not the best headphone I own. If I could keep only one I would take the HD6XX over the Aeon I think. (if I would only consider tonality I would take the Moondrop Blessing 2 Dusk over anything else I own

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The Aeon has a pleasing sound and is very detailed, but is lacking in the "fun" category. It has has poor dynamics if you listen at medium loudness levels. There's no slam to the bass ...at all. When you listen at high SPL levels it comes alive more. Mids are neutral to bright. But certainly not overly bright, to my ears. It's not annoying in any way. Lots of details but maybe lacking just that little bit of warmth the HD6XX does have.
But in general it sounds really clean with lots of details. If you are looking for details, want to hear every little nuance in the recording, this is a very nice headphone to do that. It has a nice balanced sound with maybe a little bit of extra brightness. If gives you a much more open feeling to the sound than the HD6XX, which is surprising considering this is a closed HP. It has a stunning build quality and is quite comfortable to wear.
The HD6XX is more lush. More intimate. It has more punch/slam/dynamics in the lower end (but a little less extension) compared to the Aeon RT. Again, very contra-intitive if you look at the designs and measurements of these headphones. It lacks some detail in the treble that the Aeon clearly does reveal. But not much. It's just a little more in the background. Mids on the HD6XX are it's main strong point. Soooo, soothing to the ear. Everything about them sounds right. Build quality is very decent and comfort is super, for my head and ears.
The K371 is a bit a combination of both but less refined. It had the bass extension and the slam. But maybe a bit too much of it for some (not me). Vocals are clear but maybe a bit to much elevated. (just a little too much pina-gain?) It has a very enjoyable fun sound. Very good for not too critical lsitening but just to enjoy the music, maybe during other activities. But You could use it for critical listening too. It just is that little less refined compared to the Aeon and HD6XX to my ears. Build is nice. I have nothing to complain about with my K371, but others opinion differ on that. Comfort is OK. It's light and fits nice on the head and around the ears. The pads are OK but a little sweaty.
I like all 3 more than the other headphones I own. (I forgot about the Sony MDR-ZX770BN, which is a reasonably good BT headphone) Each one for it's own character.