Cable could make a difference, but it will be subtle. You will still have the brightness problem, but possibly to a lesser degree. It's kind of like using duck tape.
Power Conditioners to reduce Sibilance
I moved my system into a new home and was having problems with treble brightness and sibilance. I moved the speakers around and got rid of most of my troubles. I then upgraded to a more revealing preamp with money that I saved up and the problem returned (despite sonic improvements in other areas). I have read that power conditioners are great for reducing sibilance. Is this actually the case. What would you suggest for under $500US used?