Getting rid of harsh, shrill treble

Agreed with Tvad. There are too many unknown variables to answer your question. It could be the room, speakers, speaker setup, CD player or amplification. A complete list of all system components and a description of the room are needed in order to help folks make recommendations.

Enjoy,

TIC

Please describe your system, including cables

It's probably the room and listener placement (sitting too near the rear wall seems to add harshness along with the bass boost).

As others have responded, changes are system dependant..
however, I had great success using a JPS Labs digital cord powering my CD player and it greatly reduced brightness and sibilance..

I agree with Tvad that more information would be helpful and I would add that harshness can also be due to, among other things, cables, plugs, dirty contacts, power line issues, RFI, interaction between your components, need for vibration control, etc. My own experience has been that it could well be a combination of a few or even many of these factors - every one of which I have experienced at one time or another myself. I would suggest an approach where you deal with these issues one at a time in order to see what helps or hurts in your system.