I need help diagnosing a sibilant problem on vinyl


Rega Planar 2 turntable. Rega Elys 2 cartridge. Audible Illusions Modulus tube pre-amp. Audio Research D-51 tube amp. Rogers LS-2a speakers. Oppo BDP-95 disc player.
My discs, especially the SACD's sound fantastic. Vinyl instrumental's sound good. Anytime there's an "S" in the vocals on vinyl there's a harsh sibilance that doesn't make sense. It would only make sense if there was a cartridge/stylus problem or a phono stage problem in my pre-amp.
Before I take it to my very expensive, very slow HI-FI tech, does anyone out there have an idea of what it could be?
sprintz699