Hey, I own both the HD 598 SE and the DT 990 Pro 250 ohm. Being a hobbyist sound engineer for over 15 years, I want to use that experience to mention a few things I noticed about the Beyers:
1)They are BRIGHT no matter what music. They have a bothersome sibilance in the highs. It's hard to ignore or push through that metallic tinge no matter if the volume is low or moderate.
2)They lack the midrange fullness of the Sennheisers. The Beyers have a very transparent sound, where there is nothing in between the sounds, and it's impressive on a technical level, but makes the music sound like a recording, not like a musical performance. I do not think they sound thin, but they lack the organicness of the HD 598. What sounds like "muddiness" on the HD 598 is really a musical quality, not an EQ quality. Wind, percussion and string instruments in real life all have some sort of midrange "muddiness" to their sound due to the materials they're made. They do not sound perfectly high-res.
3)The bass is strong, tight and solid but empty to my ears. It doesn't have show the nuanced, realistic quality of the bass instruments and kicks. The HD 598 translate all those goody sounds properly. Again, technically good bass but musically inaccurate.
I admit I'd like the HD 598 SE to be brighter and more revealing, closer to the DT 990, but seriously, they don't need the brightness. Instruments in real life are never harsh or too sizzly in the top end. I do not recommend you the DT 990. I also own the DT 880 Edition 600 ohm and those have fuller mids but they're still metallic. It's not a musically-pleasing sound. Something real is missing from their sound. I mean this, again, in musical terms, not EQ terms. For example, a conga coming from the left ear sounds like conga in the room on the HD 598. They fool the ear and brain. On the DT 990, it sounds like a high-resolution, unrealistic recording of a conga. Technically, it's sharper, with more detail, but congas do not sound that defined in real life. They do not sound so crispy or sharp.
The HD 598 SE are my dullest headphones (besides my Porta Pros), but they are the most satisfying to my ears. I think there is something special to their sound. They sound real and satisfying even if the Beyers are more revealing and (artificially) crisp-sounding.