To answer the original question - IMHO they aint.
Spent a week with them, returning today.
They are 'nice' looking, they sound better than Koss PortaPro and, while unamped, they are at least as good as any phones I have. When amped, HD 598 are even better, and clearly better than my lesser phones. They are possibly on a par with DT 770 Pro (250), but still nowhere near K 701 or DT 880/600('05). But of course you cannot really use the last three unamped at all.
So HD 598 are really good unamped and still better when amped. As such they have an obvious niche.
The downside is that unamped they are not considerably better than old HD 515. Or Sony V6. Different, somewhat better, but not worth the outlay. Amped HD 598 are better than those two, but I already have headphones which I like considerably better when amped.
Moreover, apart from the grilles and cable, the build quality of HD 515 and 598 is substantially the same. All main elements are made of the same type of plastic. OK, HD 598 have metal grilles, I give them that, but that's about it. Who on Earth cares for fake wood and pleather and/or leatherette?? I did not mind the colour scheme, but given the materials it is not stylish, but rather tacky. And just try to extrapolate the traces of wear of HD 515 (or 555 or 595 for that) on creamy HD 598, - they do not develop patina, they start looking shabby in a year or two.
Finally, one of the reasons for new headphones was to use them with MP3 language course material - and for speech I actually prefer HD 515. So HD 515 and possibly PortaPro will have to do as the language course "workhorse".
However, I kind of enjoyed Senn sound, so I am getting HD 600 while I can (Amazon.fr for me).