Power Conditioners to reduce Sibilance

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.

Not sure if the power conditioner would tame down brightness in your system. When I installed Shunyata Hydra power line conditioner in my system I noticed overall reduction in noise floor, but I can't say that it reduced sibilance. You may want to audition different speaker cables and interconnects to begin with. B&Ws are pretty critical to upstream components, so if any of your components has a tendancy to be bright, it will be shown by the speakers. Also, you may want to try corner busters from Echo Busters or any other company that makes a similar product. Aside from that I don't know what else to suggest. However, be carefull with power conditioners as few brands and types actually can hurt the sound of your system. I know 2 brands that won't probably make it worst - Shunyata and Richard Gray. Steer clear of strip power bars.

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.

Let the B&W bashing begin.
I don't agree with the problem being casued by B&W speakers.
These speakers are revealing, that's all. That could be a disadvantage, but you really do want your speakers to be revealing, don't you. The whole idea is to match other components properly. Digital components cause extreme sibilance, not the speakers. Besides, B&W speakers like to be bi-wired, preferrably with a shotgun run of speaker cable of decent gauge in order to have as less impedance as possible. Matching cables both ICs and Speaker wire is a challenge. I would first look at wires that integrate well with your components and this way you will avoid all the brightness. And also your room....Just my opinion.