what causes the sss in words like 'thisss'?

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.

-IMO

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.

-IMO

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