phonostage paradox

Any comments regarding an issue with my backup Parasound Z phono preamp would be welcome: When I use a MM cartridge, all is well; when I use a MC [in MC setting], a hum develops which resists all grounding strategies. However,my Denon 301II LOMC plays loud enough even in the MM setting, as does a LOMC Benz Micro Gold. Any explanations???
The signal to noise ratio in the MC section is not nearly as good as with the MM stage.
What happens when there is no cable hooked up to the input of the preamp? Does it hum then?
Atmasphere: The hum only occurs when hooked up to the preamp,
and then only in the MC setting, not MM.