Parma is the only thing that specifically says it can be lowered. But one can't take that as a blanket assurance that no other form of MR can be lowered. There's nothing even hinting that no creatures of might could lower their protection. In fact, IIRC the Goetic sorcerers from RoP: Infernal would run into all sorts of issues if they couldn't lower their MR (that's just a thought, I didn't look them up to double check).