Remember always that what you read here are personal opinions (including my own!) that may or may not reflect your experience. The only thing that really matters is what you think. However, there are a couple of statements in this thread that are patently false, and need to be corrected.
"If I'm not mistaken it's 4 or 5 ohm nominal, but drops well below that for some frequencies."
Maggies have low impedance (4 or 5 Ohms) but it's a very steady impedance. It never drops below 4 Ohms! It basically resembles a 4.5 Ohm resistor modified by a crossover, leading to a peak at 600 Hz. Any amp or receiver designed to handle a 4 Ohm load will drive Maggies without problems.
"They also have an incredibly narrow soundstage."
Nothing could be further from the truth. Maggies have incredibly wide soundstage that will put the majority of boxed speakers to shame.
There is also a widely perpetrated myth that Maggies are lousy speakers when it comes to dynamics. Maggies have fairly low sensitivity (about 86 dB or lower), and will need a lot more power to reach the same SPL relative to your Klipsch speakers. However, given enough (!) power, Maggies will be very dynamic indeed. The size and layout of your room, as well as your personal preferences will play an important role in determining how much power you will need for Maggies. What do you like in movies? Do you want to be blown out of your chair by high sound pressure levels, or do you want startling realism that will make you grin in disbelief as you turn your head around to look for sound sources.
I suggest that you try Maggies. You can do so at no risk directly from Magnepan. You may want to replace MMG-W with MMG for L-R duties down the road, and move MMG-W to surround location. If you have enough money now, I would recommend getting MMG right away. That speaker cannot be beat for the money or even twice the money. You may find out (like many other people) that Maggies have uncanny transparency, soundstage, immediacy, transient quickness, and just plain simple sound more like live music than many speakers costing several times more.
Your receiver (XR-57) should be fine. The lowest subwoofer crossover setting is fairly high at 80Hz, which means that your Maggies will not have to be driven very hard, and the receiver will not have to struggle too much. With MMG-W and MMG-C, you will likely have to set the crossover at 120Hz. If you don't like your set-up, you can simply return the speakers to Magnepan. Based on my experience, I don't think you will.