What is the cause of my sibilance problem?

Sounds like RF interference and/or dirty AC power.
Try a PS Audio P300 on front end or at least a Monster HTS-2000 power conditioner. Make sure you're using shielded interconnects and Speaker cables. -Ed

I appreciate all of your responses and apologize for not posting earlier - my ISP was down for two days.

When I originally auditioned the Super Silver interconnects I liked everything I heard except for the top-end. At the time I owned a Roksan Caspian CD Player. Adding vibration control to the system seemed to smooth things out to acceptable levels. Perhaps it was just wishful thinking.

What have I auditioned in the way of interconnects in addition to the Super Sivlers? I tried Wireworld Equinox III+, Harmonic Tech Truthlink, and AudioQuest Viper. Of those the Wireworld was the best. But all of them had the same sibilance problem. I only audtioned a single pair of each. Perhaps the suggestion to audition multiple pairs of each is a good one.

I forget to list my PS Audio P300 power plant and Quiet Lines in the list of equipment. The only components without conditioning are my power amp and subwoofer.

I usually listen with the cable TV unplugged. Maybe this isn't enough. I'll look into it. However, I live in an apartment it may not be feasible to disconnect anything.

All components except the power amp are sitting on brass cones. My digital front-end ( DAC and transport ) are also on shelves supported by cork/rubber feet. In addition, brass weights sit atop everything except the power amp. All of these vibration control devices are form Mapleshade.

At first, I thought my power amp needed servicing. I sent it B&K in New York and a few weeks later put it back in my system. After 200 hrs of burn-in the problem hadn't been solved. A call to B&K revealed that they hadn't found anything wrong. As a side-note, B&K technical support was outstanding. They only charged shipping and replace the top cover with one with better ventilation.

I plan to try the new Antique Sound Labs AV-8 tube monoblocks. They may not be the best, but it fits my budget right now. I want to hear the tube sound everyone talks about and see if it might get me a step closer to eliminating this problem.

Again, I appreciate everyones comments and plan to try them.

Gallaine, is it possible that your source material is the problem? Performances of close miked female vocalist, if not processed fully, will tend to be somewhat sibilant. This can also be the case if the singer is not familiar with the recording process. Is this problem on all your music, or only on a select few discs? It's unfortunate, but as you system gets better and more resolving, the quality of the disc becomes an issue.

Onhwy61 - it is certainly possible that it is the source material. I don't have that many CDs with female vocals, but the problem occurs on all of the them.

Gallaine:
My question regarding the use of cones is a key one.
Please advise which brass cones from Mapleshade are you using and how specifically you have them set in your CD player and DAC. Where and how are the cones positioned?
Do you have one of the cones at the rotation axis of the disc? Are they point up or point down?
This cone detail has a great doing with overly enhanced highs in my experience.