Phono amp impedance

I have a Fosgate Signature which has settings for MM and MC. It also has a knob for matching load impedance of your cartridge.The cart is a newer Ortofon SPU Silver Meister. When set in MC or MM mode there seems to be no discernable difference in sound when knob is run through all of its impedance settings.
sorry this thread should have been listed in ANALOG.
Check this thread out in the Analog forum. There is some good information regarding impedance settings toward the end that might be of interest to you.