Sennheiser HD-26 Pro

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.

The HD26 Pro are similar to the classic HD25 with 2 main differences.
1. They are more comfortable on my head and the automatic headband splitting mechanism works like a charm. You can eliminate it with provided single band if you got used to a single headband.
2. The hyped sub-bass frequency response is something that needs to be considered. If you work with inaccurate boom operator you will hate him for nothing as you'll hear excessive handling noise which is not really that bad if were auditioned on a regular HD25 headphones.
At the same time it could be a great benefit if you need to compare two sources looking mostly for the low end.