No damage will occur in either case.
Your cartridge was designed to be used with the MM stage. The MC stage is louder because the gain is higher to accomodate lower output MC cartridges. Since yours is high output you don't need the extra gain from the MC stage.
It really isn't 2 different stages anyway. When you switch to MC it is inserting an extra gain stage that then feeds the MM stage. As long as you are getting enough volume with the MM stage then you are probably better off since it goes through less stages, and as you noted, it has the proper load for your cartridge.
Your cartridge was designed to be used with the MM stage. The MC stage is louder because the gain is higher to accomodate lower output MC cartridges. Since yours is high output you don't need the extra gain from the MC stage.
It really isn't 2 different stages anyway. When you switch to MC it is inserting an extra gain stage that then feeds the MM stage. As long as you are getting enough volume with the MM stage then you are probably better off since it goes through less stages, and as you noted, it has the proper load for your cartridge.