The Re 75 is not powerful enough to drive Magnepan speakers properly. They need power to sound their best. I own a Ref 110 and wouldn't pair it with Maggies for the same reason. I have a friend who has the 3.7i with 2 bass panels paired with Pass 60.8 amps. This is nice setup. But at 2 0hms with the bass panels, the Pass is putting out around 240 watts before it even leaves class A.
The ARC Ref 2 and Ref 3 are very different. Starting with the Ref 2 mk2 ARC design their pramps around the 6H30 tube instead of the 6922. This had a huge impact on the sound. People either love this tube or they hate it. I like it.
The Ref 3 is probably the sweet spot. As mentioned their is not that big a difference between the Ref 3 and the Ref 5. The Ref 5se is a huge step up from the Ref 5.