what causes the sss in words like 'thisss'?

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.

-IMO

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.

Jeff