Coming from the classic HD25s, this is a very clear upgrade. There's very few reviews online about it, but those saying the two headphones sound the same, they really don't. The HD26 improves on the HD25s by a lot. Sound separation and depth is very evident. Bass is not as strong. When comparing both, I can only summarize as the HD26 sound very clear and precise and the HD25s sound muffled, like there's a wall between you and the audio. You can also drive these headphones a lot easier despite it having 100 ohms. I keep my headphone volume on my Focusrite at 25% or below.
One thing I disliked was the auto opening headband as it could never fit my head right. Luckly they do include an option to change from the auto opening one to a unified single one. So if you're on the fence of getting a pair mainly for that reason, they give an option to change headbands. Another dislike are the on ear earcups. That was to be expected before buying. I never particularly liked the HD25 earcups either until I changed the earpads. Something to keep in mind.