Really hard to say anything against these headphones. Great comfort (after 18 months), easily replacable cable/earpad in case of natural wear. I can use them for extremely long sessions without any fatigue or heat building up.
The EQ is pretty flat which is what I like about headphones and They are perfect for music/gaming and movies alike. I use them with a creative G5 sound card/dac and a modmic for discord sessions with friends.
Can't compare to the Sennheiser 6XX but I can imagine they will work as well.