what causes the sss in words like 'thisss'?

I run apple lossless codec -> airport express -> recently purchased musical fidelity x-dac & tube buffer -> rotel 1056 receiver -> Quad 22L floorstanding speakers . I have also tried a cheap cd player source but no luck.

So what causes those annoying sss sounds in words? Source? Amp? interconnect? I'm considering returning the musical fidelity stuff and spring for a naim CD5 and nait 5i combo. Thoughts?
Posssibly room reflections. If there are lots of hard surfaces to the sides and rear of your speakers you may need some treatments to prevent reflections too close in phase to the leading sounds. Nothing fancy just try hanging a decorative rug or favorite crushed velvet painting of Elvis near the speakers.
It's called negative sibilance and it's almost solely a result of dirty AC from the street coming to your home. Dirty AC can also come from your appliances, dimmers, and your digital dac or cdp.

However, proper line conditioning can fully resolve this issue and more.

This weekend I got a naim-recommended power strip - the wiremold L10320 - http://www.naimusa.com/2004_site/html/power.html

Is there a power conditioner you'd recommend instead?
Congrats, you have a great system. I noticed the same thing with mp3 sources. I think its a poor source coupled with high resolution playback where you have no high frequency rolloff.
Sorry, I forgot to mention that proper line conditioning can only rectify the negative sibilance induced from dirty AC.

Obviously it cannot eliminate negative sibilance permanently recorded into the medium.

CD players.
This is digital sibilance. You probably hear it most strongly on female voices on CD recordings. I found the best way to get rid of this (I too hate it.) It is sooo anoying is to;
1) Buy a more expensive CD player/digital source with better anti-jitter, clock, and DACs and that is tubed, and;
2) Go to tubes from solid state gear, or;
3) Go back to vinyl (best option IMHO)
U2's "crumbs from your table" for an example is really killing me. It's one of the better songs off the new album and bono is making my ears bleed (some may not think that's abnormal). Does anyone else mind listening to this song to see if it's the recording?

It's the album. I am a U2 fan, but the last couple albums (not that U2 albums are "Audiophile") have been poorly recorded. Great music though!

You are more apt to hear the negative sibilance on female vocalists than anything else simply because females (and a few males) have extreme precision when pronouncing the 'ssss' sound.

And it happens to be one of the most difficult sounds to reproduce accurately.

For some reason, this is the area that is most notably improved once proper line conditioning has been installed.

And it sure beats listening to all that sh'ing noise.

In my experience, I've not noticed this negative sibilance having anything directly to do with the room acoustics or digital sources. But I suppose it's possible.

Get your favorite crushed velvet portrait of elvis regardless. It cant hurt...that much
I think it's many reasons combined.
1) clipping at high freequencies
2) poor recording that again might result poor equipment to clip at high freequencies not neccessarily at amplifier/speaker domain but adequately at source or preamp.
There's something else to consider. Either the recordings microphones were improperly placed or the wrong style microphones were either properly or improperly placed,or both.

Does the severity of the problem vary from recording to recording? Does the severity of the problem vary with the locations of the instruments within the soundstage-indicating distance and direction from a microphone?

The right interconnects can help. Switching from some old Synergistic Research stranded silver interconnects to DiMarzio M-Path eliminated negative sibilance in my system.

I am curious though, what power conditioner do those of you who suggested that recommend? The next upgrade in my system will likely be a reasonably priced power conditioner.

AC line conditioning helped cure sibilance issues I had in one of my systems

1. Place the microphone setup... and blow onto microphone.
What will you hear from speakers?
2. Repeat blowing onto microphone only with joined teeth pronouncing an "S" sound and you should hear exhagerated sibliance.

Sibliance in RECORDING is a result of noise caused by the wind onto the microphone.

The recordings that do not have any sibliance have an advanced filters and compressors to eliminate this problem during mastering proccess.