So Much "Harshness"

Assuming everything is in good working order to start, then most often either because downstream electronics cannot reproduce the sharp transients found in many digital sources cleanly and/or amplifiers are not properly matched to drive a particular set of speakers properly and/or speakers not matched to room.

Noise/distortion present or introduced into the line level signal can exacerbate the problem certainly as well.

Also I believe it is seldom due to the recording other than that some digital recordings are hard for systems to reproduce cleanly again due to the sharp transients present. That is more of a problem with the challenges of playback than it is with the recording itself.

A few strategically placed tubes can definitely help I have learned of late. They seem to act as natural and highly effective high pass filters that definitely take the edge off things in avery natural sounding way.

Oh, and lets not forget about jitter in digital playback systems, though I think this is less of a real problem in general with newer equipement these days than it used to be.

I think that pretty much covers it....

Lots of causes resulting in lots of cases as the poster observes.....

Harshness is a very general term. It is also a very common problem. It can result from lots of things as has been pointed out.

So the devil IS surely in the details when it comes to curing harshness. There is no single universal cause or antidote.