Treble harshness - interconnects as culprits?

The usual culprit in harshness is the source.
Most decent 'down from the source' are subtractive. If they have a bad thing it is usually not added to what comes in, rather it takes away a bit of something.
(IE: why I use a tube buffer between my DAC and pre, to not have such a pure HF passthrough, to cut the digital nasties in my system).
So I would say if you are getting treble harshness it is from your files on the computer. The DAC is probably just responding to the files, or the DAC is picking up EMI stuff from the computer
On the other hand, Do you use a powerline conditioner? (Mainly on the digital items) And is the connection USB, toslink, or RCA between the computer and DAC?
I would try a toslink, to cut the electrical connection between the computer and the rest of the stereo.
Then, try a small powerconditioner, like from Audio Advisor (where you can try it and if it does not work, return it within thiry days full refund, They send it free if over $200. so your only loss would be return shipping)
You could try different interconnects, or speaker cables, but I dont think that would alleviate your problem.

Additional thought: perhaps your computer is processing the files differently than before. I know I can accedentally reset something and not know it on my computer. Perhaps the way the files are being used has be changed?
Anyway, it might be a reason to check how your server is processing the files going to the DAC.

I would get rid of the RS interconnects anything would be better choose one. Try kimber PBJ. Any Kimber stuff is good.

Elizabeth makes some very good points and doing the things she suggested should all help.

Other considerations are that there is a very good possibility that the sibilance does exist in the recordings themselves and your system is simply showing you what is on those recordings. Also, different cables do sound different and some cables and certain combinations of cables (say interconnects/speaker cables or speaker cables/power cords) can sound more or less sibilant than others.

So being the somewhat frugal beast that I am, I would try different interconnects and/or speaker cables first (especially if I had them on hand). And if you don't have a good AC power conditioner in the system, I would get one... not only for better sound, but for protection against spikes and power surges.

Frank

The only culprit here is the power. You would need high quality power cords like a Transparent Audio Reference power link or a good Regenerator like a Pure power or a Monarchy. I have gone through this exact situation. Any attempt to cut down harshness through interconnect will only lead to coloration and softening of transients because the problem doesnt exists there and you are trying to forcibly cure it there. POWER POWER POWER !!!!