It's possible that the sibilance you're hearing is an accurate portrayal of what's on your CDs. Sibilance can result from bad miking, poor microphone choice, the vocalist and/or over processing of the music signal. Most of the above post only address the upper midrange hardness that may be part of your system and they will do nothing to eliminate the sibilance if it's in the source material. If you're truly serious, you need a de-esser (a frequency dependent compressor). Consider the TC Electronics Finalizer line or Drawmer Masterflow Processor. Both are pro audio signal processers that will eliminate sibilance. Most likely you don't need to get this serious, but if you do, these are the components you should look at.
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.)