I own both the shp9500 and the x2 and your description of the treble of those two headphones is in line with my impressions.The 560s are not even remotely close to having harsh treble.
Something like the shp9500 are what I consider to be harsh treble. Those definitely have lots of sibilance. They can be piercing as hell with certain music and games. The x2hr has harsh treble in that it's sort of peaky and all over the place.
The treble on the 560s is clear and precise, I haven't heard a hint of sibilance. I'm not sure what role my schiit hel may play in how the treble is presented- I doubt it has an impact here.
If the treble of the 560s is more clear and precise that's a good thing.