brightness in my system

First change your cables, especially loudspeaker cables. I recommend you MIT terminators. You will elliminate most of the brightness. Another thing is, if your source is digital, most probably you have a lot of jitter, go with Monarch DIP jitter filter(very cheap but fantastic) or go with Genesis Digital Lens (expensive but valued), you will be ending with majority of your brightness

I have a 15k system with MIT cables and as the other post suggests, they will improve your system.An extremely low cost tweak would be to get a CD Blacklight mat.I have one and confirm that it will take smoothen the edge of off highs

need to know the components in your system to properly diagnose the problem. are you using a preamp?