Sounds like you're describing the diagrams here:
https://www.rtings.com/headphones/reviews/sennheiser/hd-660-s
Specifically the Bass/Treble accuracy diagrams. I have a pair and I think they're great but I don't disagree. I don't know how much is the headphones and how much is my hearing (can hear ~17kHz). I ended up going old school and getting a Schiit Loki to EQ them as I couldn't face faffing around on the computer (I use a headless Pi). I really like the Loki. Anyway, I've added a very slight bit of boost to the bass (the 20hz knob) and seasoned the treble (using both the 2kHz and 8kHz knobs) to taste. Fairly unscientific but I put together a playlist to cover a number of genres, including pieces I thought were a little dark sounding before, and spent some time a/b-ing it and I'm happy with the result.