It's a pretty extreme example. It makes sense that his ears are sensitive to treble. That means the 6XX balance the ears out, who knows.
What this U or V shape discussion has me thinking was the Fletcher-Munson equal loudness countour curves. At low volumes, you need more of a elevated bass and treble response and that alleviates as you raise the volume.
A lot of these imaging discussion can be caused by confusion with recording's characteristics.
Of all the other headphones I can point out what significance in sound I've heard. LCD2C, upper-mids dip, lifeless cymbal energy. Sundara, male vocal mids dipped. HE560, 7k prominent bright. HD800(S), 6k beyond sustained high treble relative to lacking bass. Elex, as bright or brighter than Utopia even due to comparatively lowered upper-mids and lower being at a lower level.
Just like the 660S, 58X doesn't get to such extremes. 58X comparatively has more elevated bass, and sounds more dynamic. 660S hasn't had much dynamics was the caveat. Quite balanced, and not dark like HD650. Close to neutral, but these two mids male vocals may sound slightly lower than neutral, or distanced, but nothing significant.
58X is so tight, clear, and quick in response. Perhaps it's my solid-state amp with such quick drivers, dunno. Very airy, and I'm havin a blast listening to it. I've not been so motivated to explore music. Tidal helps out as well. 58X got me deep in music that's it's probably not healthy. LOL.
I would say characteristically similar in balance would be the Focal Clear.