My Denon AVR-5700 had the same problem, I changed speakers, tried various interconnects, changed power cords, and my best results at taming those "SSSSSS'S" were the Harmonic Technology Truthlink interconnects. When I can afford it I will upgrade the Denon to Theta gear.
smoothing those sibilants...
I appreciate the feedback on the interconnect post I made a few days ago, and here is my next question: If I am attempting to smooth out the sound of a "low-end-of-the-high-end" system, reducing grain while retaining detail (and looking for a warmer more "classic tube sound"), where is my energy/$$$ best spent? Would it be the digital source itself, the interconnects, the input tubes, the speaker cables, or the speakers? Or something else? (System info can be seen under the post entitled "need interconnect advice", and there are some new interconnects on the way.)