Impressions after a week or two with the Neumann's:
Weird headphones. Not sure if I love them or hate them. First thought after about 10 seconds of listening: They sell these as neutral monitors? They are NOT neutral AT ALL. Very boosted bass and rolled off trebles. But after 2 weeks, I have to nuance that first impression: I still think they are far from neutral (whatever that is), but for me, they work with some music, and not at all with other music:
What works: Older music, mastered in the 90's and before. In those time, tracks were being master in a "thinner" way, less dynamic and less bass. Old rock, pop, metal etc. works beautifully. The NDH 20 elevates the recording's thinner bass beautifully and dampens the sometimes harsh highs of those types of recordings. I genuinely have to say that I enjoy my collection of older music much more than ever.
What does not work: E.g. recent, well mastered EDM. Tracks that already have a strong bass component are un-listenable. Total bass overkill, especially, if the recording's bass contains more mid-bass than sub-bass.
In a nutshell, older, crappy recordings sound great, well mastered, modern stuff doesn't. Since a large part of my collection is old rock etc, I will keep the NDH 20 for those, for the pleasure of rediscovering older songs. And also, because I really like the build (like a tank) and the fact that they are quite portable for a full sized headphone.