Another possibility is the recording itself. Often vocals are VERY closely miked which can magnify the sibilance. This is true of many rock, pop or new country recordings. Add to that the "loudness wars" recording style that still hasn't gone away in many recordings, or a variation thereof where some popular recordings are mixed and equalized in a manner that makes them stand out when played on so-so equipment like car stereos, cheap radios, ear buds and the like. These effects can really sound nasty when played on a full-range stereo at louder volumes.
So, the question is: does this problem happen with all recordings or just certain ones? If just some recordings, I think the above comments might explain the issue. If all recordings, maybe it is the speakers, or the acoustics of the room, or a problem with the amp. However, don't forget to look at the source, whether phono, CD or streaming digital. Could be you have the wrong cartridge or phono preamp for your system, or a nasty sounding CD player or DAC.