The MM651UM is the ultra-marine grade that is indeed has 4 ohm impedence. Right now, it's between these and the Infinity Reference speakers for my fairing.
The UM speakers have a molded non removable grill. You won't be able to put those in the fairing with that grill on there. Are you using the 2 or 4 channel amp?
I would like to get the 4 channel and run the MM651 but I'm worried about the 2.7 ohm rating.
I'll be running the PBR300x4. Its shippin now. Thats a bummer on the grills. I have the 6912m's that I'll be running in the rear bags and they have that too. I didn't even think about the non-removable grills, the 6912's are the same but that won't hamper the installation for my bags.