I'm thinking that Curtis (and others) may be right. It sounds like you're going to need to move up a notch or two for the sound you're looking for, since all the speakers in your budget range are sounding similar regarding the "sssssss" frequency. Unfortunately, your budget would have to increase for a speaker with a top-notch dome or ribbon tweeter that will sweeten the top end.
Just out of curiosity, though, can you name some of the CDs that give you problems? Do you get the same results listening to an audiophile quality CD? Because if you do, then something other than the speaker could be at play (as suggested by others), since I had never heard sibilance on the 170s from well recorded CDs.