Hey Guys - I am trying to figure out which sub to get to go with my System. I have 3 MPX in walls for LRC and 650's for the surrounds powered by an anthem 510. I was all set to get the FF4 or 5 and then heard the the Logan which I can get for about the price of the FF5.

Any thoughts?

Thanks!

I'm not a huge fan of mixing and matching speaker brands (personal opinion) in a system. I like to pick a brand and stick with it for all speakers. With that being said, I have not heard the ML sub.
How big is your room? In my experience A single larger sub driver tends to fill a large volume room better than multiple smaller drivers.
My advice is to listen to both of them in the same room with music/movies you are familiar with. If this is not possible, Andrew Robinson has reviewed both speakers (FF4 not the FF5).
hometheaterreview.com/martinlogan-decent-i-subwoofer-reviewed/
hometheaterreview.com/goldenear-technolo...-subwoofer-reviewed/

The ML sub is about 2-1/2 times as expensive as a ForceField 4, almost twice a ForceField 5. If you can buy one of these for 1/2 price because they are being discontinued, that's pretty hard to beat. But, as the review said about the ForceField 4: "the ForceField 4 Subwoofer from GoldenEar is a tour de force among its peers and even costlier competition". Dollar for dollar, ForceField subs are unbeatable. Half off the normal price on ANY other sub probably evens up that equation.

Also, if it matters, the ForceField is a lot smaller. My wife likes the GE subs in the house a whole lot more than she liked the big ugly boxes I've had in the past.