I'm glad changing the loading helped a bit with the sound.
I have a feeling the diamond may be mounted slightly askew causing one side to be sibilant more than the other side.
It may be worth taking it to the dealer to be checked against another cartridge.
Something doesn't sound right to me that it's always one channel causing an issue...
Or, you really need to get yourself a cartridge with a microline or Fritz Geiger Stylus.