Finding source of sibilance

More info needed.

First question, as slaw is saying, does your system have sibilance problems when using sources other than the turntable?  If so, more information is needed.  If not, you might check the alignment
or VTA of your cartridge, that could be a cause of the problem--in my setup, for example, if the cartridge is set up so that it is not parallel to the platter and it is instead leaning forward (i.e., the back is higher than the front, looking from the side), the highs are accentuated. 

Sibilance can be caused by even a small degree of cartridge misalignment. But it can also be caused by the stylus being attached to the cantilever slightly "off" by it's maker, or even the cartridge being run with too little tracking force. A qualified turntable setup man in your area is a blessing! If you provide your location, perhaps someone can recommend one.

Assuming your system is set up properly (im sure it is) how long have you had this equipment? It takes roughly 100 hours of burn in time before things open up and sound right.Try not to jump to worse case conclusions right away, its probably something simple. MM's like to have the correct capacitance set, so make sure you include the cables capacitance too, this can be 50pf or as high as 250pf! try and keep cable short (2 ft) to reduce this.

Matt M