I was in the same boat regarding the sibilance. I can't believe it's taken me about a year to really enjoy these. Not sure if its psycho-acoustic or that they've started to burn it (very little use over the past year). Part of it was fit too, and I can imagine everyone's ears are going to sound different. I use the ML hybrids but have just put in the large isolations buds.
When I first got this phone, I though it was so harsh. What I know is that if I want to listen to something with some serious punch (ie. psychedelic trance) these are it. They seem a tad light on bass quantity but the quality is so good and clear. Then you turn it up a bit more, and everything is perfect. I feel a bit spoiled
brand new MDR-1r is being ignored right now. I was considering selling/trading the MDR-ex600 with the arrival of the MDR-r1, thinking I need an open can. Was looking into Hd598 or 558, but I'm realizing there are great qualities to this phone, and while not perfect I can get a lot out enjoyment out of them.
If I was to get a headphone, is there one out there with a similar sound signature, with maybe just a bit more bass?
Update: July 26th - had an odd issue with ex600. I could never get a proper stereo field, as the right channel always sounded a bit louder. Not sure what I did but I detached and reattached the cable and huge improvement. I had thought it was just a seal issue but I didn't have the best connection. Also the MDR-1r went back. They are nice headphones, but I just didn't need them.
Anyhow, now fully and properly enjoying these headphones.