In the meantime, I set up my turntable.This is a Technics SL1200Mk 5 with an AT150MLX cartridge.
Turntables, especially those with high output, can have a variety of issues causing sibilance. Make sure you have a low capacitance cable between the tonearm and preamp- that will help prevent brightness. The cable should not be more than a meter and 12-20pf per foot is a good value for capacitance of the cable.
The mechanical setup of the cartridge in the arm (VTA, tracking angle and the like) also can affect its tonality. The platter pad can too- the rubber mat found on most Technics machines isn’t helpful. This is all probably a topic for another thread, but since it all affects sibilance I felt I should mention this.