To quote the manual for my Klyne SK-5A preamp, "The effect of loading a cartridge with a relatively low value of input resistance is to provide some dynamic electrical damping of the moving coil generating assembly. In general, a higher input resistance will give relatively better signal to noise performance, whereas a somewhat lower value tends to reduce treble distortion. Too low a value will rob the midrange of life, fullness and clarity."
Again, to quote the Klyne manual, "Many MC cartridges suffer from a rising high frequency response starting somewhere between 10KHz and 25 KHz. Typically this rise will reach a peak of up to12 dB in the vicinity of 40KHz and then fall sharply as the high frequency response limit of the cartridge is reached. This high frequency peak is audibly discernible and can contribute greatly to listener fatigue. ... it can also stress high frequency performance of the RIAA amplifier."
Changing capacitance values will change the high frequency rolloff characteristic of your phono section, and whether the Sigma is MC or MM, it looks as though your capacitance setting may be worth playing with. Klyne recommends you do it by ear starting with a rolloff at 16KHz and working up, but I'm afraid I don't know what starting capacitance value that represents--perhaps the lowest.