My Lord, you didn't read my post--'classical exclusively'. If you check around you'll find another classical listener who also posted about the 598 being harsh with strings, only he put it down to a resonance in the cups. Personally I was surprised when I first tried them, as I always liked the HD595 and never found it harsh.
As for the DT880, I don't find the treble harsh at all. Probably due to HF hearing loss I'm not bothered by the treble peak, and in terms of quality and texture I find the DT880 just about spot-on.
I did read your post. Even within classical music, you have to be specific. For instance, if you listen to cello music I doubt you'd perceive the 598 as being harsh. With the upper registers of the violin, I could see how one can come to that conclusion though. With organ music, I could also understand.
The HD598 have a more tonally accurate midrange than the HD595 though. I have the Mad Dogs to compare it to, and if the Mad Dogs indeed reproduce the frequency with a perfectly flat curve, the HD598 does have a slight emphasis in the mid-midrange to upper-midrange, but not enough to the point where it will fatigue you.
I found the DT880 to have a better texture to the bass than any of the Senns. Less "one-note" sounding bass. As for its treble, can't say. For my young ears, they're a bit too fatiguing even at moderate volume.