Where do cables upgrades have the most impact?

I put in some interconnects that's made my system a joy to listen to.  I can't tell you which wire will do that to your system if any but good luck.

It really depends on your system.  I had a hot harsh analytical system that needed some mellowing out and Cardas Golden Refs were just the ticket .   It was magic that's all I can say. I'm sorry they stopped the golden line but I bought some authorized close outs I haven't employed fully yet or at all.