Advice for RCA Interconnect Upgrade?

Cables as tuning instruments is a losing proposition.  Upgrading cables is always a good thing.  However, don't expect it to cause less response in the treble region.  You'll have to make other changes there.  Do you hear harshness or is this just an app causing you distress?  

I'd look at the modifications you've done to your speakers and the potential loss of linearity.

Jerry

Invest in room acoustics first if you haven't. Super easy to tone down a bright system with absorbers.

I've gone for a full loom of Audiomica (previously Cardas) and have since stopped looking

Take a serious look at Wireworld. I have all Wireworld silver eclipse 8 XLR and RAC cables. Plus all Wireworld power cables. I learned to stop mix and matching cables. I started to upgrade my second system to these as well, the nice thing is I have them now so I can try them on the other system and hear the difference. I had a startle 8 red cable from my CD transfer to the dac, and swapped in the sliver eclipse8 and the difference was huge. All the shrill from the Norah Jones CD was now gone, rounded off is you will. The clarity and articulation is more pronounced.

I little work on your room can go a long way to “Taming” your room.