Do I understand you correctly - you are plugging the RCAs from the turntable (actually arm/cartridge) into something other than the phono stage in your preamp (or a separate outboard phono stage)?
If so that is your problem. An MC cartridge generally has a has a lower output than a MM cartridge and needs additional amplification. Its capacitance usually needs to be much lower as well, but both types of cartridges need a matching phono stage. You cannot (successfully) just plug the phono cable into a line input of pre-amplifier to get the necessary equalization needed to run any cartridge.
You can use the MM input with a MC cartridge using an outboard amplifier specially made for this purpose.
If so that is your problem. An MC cartridge generally has a has a lower output than a MM cartridge and needs additional amplification. Its capacitance usually needs to be much lower as well, but both types of cartridges need a matching phono stage. You cannot (successfully) just plug the phono cable into a line input of pre-amplifier to get the necessary equalization needed to run any cartridge.
You can use the MM input with a MC cartridge using an outboard amplifier specially made for this purpose.