I never fail to be flummoxed by the HD 6XX: these headphones always sound off to me when I first put them on. The slightly muffled (or veiled) sound accounts for this, as does their laid-back quality and mid-bass hump. In other words, these characteristics immediately strike me as departures from what the music should sound like - at least, according to my expectations. But, at the same time, the timbre of instruments sounds right. So, my brain is dealing simultaneously with contradictory signals; something right and something wrong. My guess is that the brain moves swiftly to raise flags about somethings being off but then latches on to what’s right and gradually adjusts to the deviations from “accurate” sound to the point where you forget their “inaccuracies.” Only with practice and critical listening - and reading the forums! - can I begin to be conscious of what these things are.
I also suspect that expectations have a lot to do with this: with a closed-back, bassy headphone like my TH-X00 Ebony, the bass always comes across as a bit of a surprise. But, then again, I think I adjust pretty quickly to them because I don’t expect them to be “realistic.” By contrast, my go-to headphones these past few months, the Focal Clear, immediately sound right to me. As a listening session wears on, though, I begin to notice a few things that sound slightly artificial (a bit of bass heft, for instance, or a slightly steely or glassy quality to the treble).