I would also check that your speaker drivers are torqued correctly. Snugging up the screws on several of my speaker's drive units has alleviated small issues.
I had this issue with some vinyl as well. I adjusted the azimuth and added a better isolation system to my TT. This solved the sibilance and slight white noise in one channel.
I had this issue with some vinyl as well. I adjusted the azimuth and added a better isolation system to my TT. This solved the sibilance and slight white noise in one channel.