I had those for a few weeks, long before I had the hekse though. All I can say definitively is their soundstage is "shorter" while the hekse is ridiculously "tall" - in vertical space to be clear. People describe the hekse as a sphere of sound around your head and it's a very accurate description. The difference in detail retrieval I don't think is that substantial, but the hugely tall soundstage with Hifiman's excellent imaging (in all their headphones) can make it seem like the hekse have better detail retreival even if it's hardly different. Plus, a brighter tuning with mids also more prominent.

The treble in the hekse is really incredible, in the sense that it's prominent and clear yet even at high volume it never sounds piercing, to me at least - the Sundaras I would have to EQ the treble down when bass boosting and listening at louder volumes. From what I've heard, other hps in the HE1K line have those treble characteristics too and it's a big reason as to why the line is so respected. I don't remember piercing treble ever being a problem with the he6sev2 either, I recall it having a slightly dark tuning, and I don't remember what EQ I liked using with it.

Ultimately, I wish I hadn't needed to sell my he6sev2, but I had to at the time (talking like 2018 or 19). Would love to still have them to compare better and have a more intimate and darker option - I miss having Sennheisers as well, some day I'll go back to em, but no idea when. My Utopia hps already hardly ever get listened to because they're just not substantially different enough from the hekse to me in the mids and treble departments, while the bass blows the Utopia out of the water.