I don't think you can claim something like "The HD598 is much better in my opinion" if you've heard the DT990 briefly. DT990's are in a diffferent league. The HD598's and DT990's are not directly comparable at all, because the DT990 is on the level of HD600/650's. It's technically superior to HD598 in every way. It has a higher resolution of sound, more detail (more even than K701), better extension, better separation, better dynamics....pretty much, it's a superior headphone in every way. In a direct A-B comparison it would take even a non-audiophile person exactly 4 seconds to tell that DT990's are the superior headphones. They just sound. look and feel like a headphone that is considerably more expensive. HD598's are good, but they're not on the DT990 level. Some might prefer the HD598's sound signature, but DT990, especially the PRO version, is very neutral, possibly more neutral than HD598. It's the headphone in BD range that comes closest to sounding as good as the T1's imho, with T90's being way too bright and DT880's have a too closed in sound and somewhat compressed / boring sound.
That said, DT990's are more sensitive about the source and amplification. Both need a headphone amp to sound good, but DT990s are definitely harder to drive. However, seeing they're actually cheaper than 598's, I think you'd have to be mad to buy the 598's. Unless you hate the sound signature of the DT990PRO's, it makes no sense to buy the 598's. Not just because they're not even near to being on the same level sonically, but DT990's are also considerably better built, more durable and comfortable. Their sound signature in the PRO version is different than with the Premium models, they're not as bass heavy or bright. They actually have a very flat sound that's still exciting, but it has that great natural feel about it. Great texture, great tonality, etc....what you'd expect from a studio headphone. It has some of the most realistic reproduction of acoustic instruments and vocals that I've heard from a headphone, and that I say with owning pretty much every German sub-500 dollar headphone and owning the T1's at the moment. Sometimes they surpass even the T1's with their "I'm there with the musicians" feeling.
Oh...forgot, about music genres. Even though I know without a doubt from my own experience that DT990's beat the HD598's even with genres in which HD598's should be superior (jazz, classical, vocal, etc.), with rock, metal and stuff like that, they just outclass them. HD598's can't even hope to match the speed, dynamics and just sheer impact and great sound separation of the DT990's, which is important in fast music so it doesn't end up sounding like a muddy mess.