I'm one of the few that find D5000 is sibilant, more than DT880 (600 ohms version), actually I don't hear sibilance on the 600 ohm version, and slightly recess mids, I can't listen to them, but quite funnily, I like RS-1 very much, which people say it's bright.
Have you listen to the 600 ohm version of DT880, since you like this kind of sound, it sound smoother than the 250 ohm one, nice treble and maybe more 'exciting' for you than the 650, although 650 is my fav phone to date.
What kind of sound are you looking at? in terms of:
- treble (Like me, I guess maybe you prefer mid to HF treble rather than lower treble, this is the one that causes sibilance)
- mid,
- vocal,
- bass (quantity vs quality),
- soundstage or depth?
- music preference? (vocal, accounstics, pop?)
- open or closed sound, they're are vastly different
- isolation needed?
with these, it's easier for people to advice.