Sibilance can be caused by even a small degree of cartridge misalignment. But it can also be caused by the stylus being attached to the cantilever slightly "off" by it's maker, or even the cartridge being run with too little tracking force. A qualified turntable setup man in your area is a blessing! If you provide your location, perhaps someone can recommend one.
Finding source of sibilance
I've recently put together a few new components. A Clearaudio Concept TT, the Clearaudio MM cartridge, and the Pro-Ject Tube Box DS. Together, I'm not loving the result. The highs sound actively bad and sibilant. Is it possible that I installed the cartridge incorrectly, or is this just not a good setup? I would assume the TT and cartridge themselves are meant to be together. Any help would be amazing!