what causes the sss in words like 'thisss'?

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

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

THE EXPERIMENT:

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.