As Elizabeth suggested, try a different loading. It is free and can make a dramatic difference. The other thing to try is to change the vertical tracking angle (VTA) of the cartridge (if your arm allows for this). Thin sound is associated with too high an angle (the arm is tilted downward from the pivot).
When dialing in a cartridge, I adjust both VTA and loading. They are sort of interactive in that they roughly do the same sort of thing, in terms of tonal balance, but not quite. That means adjusting one may change what is the optimal setting of the other. By increasing the loading (LOWER value resistor, such as 100 ohms vs. 47,000 ohms), you will tend to reduce sibilance, tip the tonal balance toward the bass (less cool sounding), but going too far makes the music sound slow and muddled and lacking in top end "air" and extension. The same would be true with lowering the VTA, but with less impact on the bass response. Some mix of VTA and loading adjustment will be the best compromise.
When dialing in a cartridge, I adjust both VTA and loading. They are sort of interactive in that they roughly do the same sort of thing, in terms of tonal balance, but not quite. That means adjusting one may change what is the optimal setting of the other. By increasing the loading (LOWER value resistor, such as 100 ohms vs. 47,000 ohms), you will tend to reduce sibilance, tip the tonal balance toward the bass (less cool sounding), but going too far makes the music sound slow and muddled and lacking in top end "air" and extension. The same would be true with lowering the VTA, but with less impact on the bass response. Some mix of VTA and loading adjustment will be the best compromise.