Gain and input impedance. The latter does affect frequency balance.
2 responses Add your response
I have a Tandberg preamp with MM and MC inputs. (In storage now). When I got the service manual, and looked at the schematics I was surprised to see that there were two completely separate phono preamps, MC and MM, but that the circuits were identical except for resistor values that set gain, and the resistors at the inputs which load the cartridge. Even the transistors were the same.
Of course the RIAA equalization is the same. As suggested by Kr4 the input load resistance and capacitance affect cartridge performance, and any difference you hear is probably a result of this.