Power Conditioners to reduce Sibilance

I doubt cables will make a difference.

Adam, I know there isn't much faith in cables changing the sound of your system, but I decided to mention this to you anyway....
Is your speaker cable a bi-wired pair?
If not you must be using the B&W jumpers? My speakers sounded terrible with a single run of speaker cable using B&W jumpers. Midrange and trebble were out of control. I changed to same cable but a bi-wired pair and I could not beleive the improvement. At least with my speakers, I noticed a huge difference. If you have someone to lend you a pair of bi-wire cables, try 'em out. See if that will change anything. It was a big and positive change with my N803s.

The bi-wire suggestion has come up in comments regarding my system and is now something I am considering. I use XLO ultra 6 and the bi-wire version (XLO ultra 12) sells for $50/foot in Canada. I may have to look into it but that is a little steep for my student budget.

You could actually find another used single run pair of the cables you already have and just double your run. Find a place that will let you audition a pair so you don't waste your $ on something that will not produce desired results.

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.