Can someone explain "Moving Iron" vs."Moving Coil"

I recently got back into Vinyl, and I bought a Sota Sapphire, with an Alphason arm and a Grado cartridge. I asked the previous owner if it was a moving magnet or moving coil cartridge - he told me it was a moving iron. I've got a Classe DR6 preamp - do I select MM or MC on the back of the pre ? I started off with MM and I don't seem to getting sufficient gain, I don't want to switch to MC, as I'm afraid the output of the Grado might be way too high for the MC phono stage. Am I being paranoid, or should I just go ahead and flip the switch to MC and ride the results? Thanks.