I've never had a sibilance issue that could be traced to cartridge break-in.
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!