Elizabeth makes some very good points and doing the things she suggested should all help.
Other considerations are that there is a very good possibility that the sibilance does exist in the recordings themselves and your system is simply showing you what is on those recordings. Also, different cables do sound different and some cables and certain combinations of cables (say interconnects/speaker cables or speaker cables/power cords) can sound more or less sibilant than others.
So being the somewhat frugal beast that I am, I would try different interconnects and/or speaker cables first (especially if I had them on hand). And if you don't have a good AC power conditioner in the system, I would get one... not only for better sound, but for protection against spikes and power surges.
Frank
Other considerations are that there is a very good possibility that the sibilance does exist in the recordings themselves and your system is simply showing you what is on those recordings. Also, different cables do sound different and some cables and certain combinations of cables (say interconnects/speaker cables or speaker cables/power cords) can sound more or less sibilant than others.
So being the somewhat frugal beast that I am, I would try different interconnects and/or speaker cables first (especially if I had them on hand). And if you don't have a good AC power conditioner in the system, I would get one... not only for better sound, but for protection against spikes and power surges.
Frank