To keep it as short as possible:
The build quality is superb (made in Germany). The material mix make the headphone overall heavier than my HD598 and 599s, but the feel of it makes it worth the weight, imo.
After a week of daily use their wearing comfort I still not quite there, although it is getting close to what I am accustomed to from Sennheiser Headphones.
Now the sound. It is absolutely something else. I really enjoyed the sound signature of Sennheiser products thus far. Warm, detailed, balanced, and never harsh, not even on the most questionable mastering. Many years ago I've tried a low impedance DT990 on a mobile player (Fiio or something). I liked the tonality, being totally different from what I was used to, but absolutely hated how harsh they became on particular songs. The detail was also not up there with HD598s, imo.
Recently I randomly came across DT880 reviews, basically suggesting they are truly best in class reference headphones, even compared to the HD600, although without the harsh 8kHz peak of other Beyerdynamic products. So gee whiz, naturally I was tempted to give it a shot. Lo and Behold - it slaps.
The 250Ohm DT880 absolutely outclasses my HD598 in fidelity, frequency range, detail, sound stage, everything. Additionally, it is almost as if my phobia of harsh peaks has been lifted at once, as soon as I heard those smooth and sparkly bright details, which my 598/599 simply aren't able to reveal.
It may very well be the magical combination with the Fostex HP-A3, which delightfully drives them at comfortable listening levels between 10 and 1 o'clock. No distortion was audible to me. Not even at 4 o'clock, which is way to loud for longer periods of listening, on my ears anyway. They definitely sound worse on my Galaxy S9, without question. Did not try any other source, however would recommend pairing them with a basic DAC/Amp at least.
I am very pleased with the new world of sound these headphone let me have a second go at. The HD598 are still fulfilling their duty, for clam and relaxed listening sessions before sleep, as my bedside headphones.