I have the 20.7's, had the 3.6's previously. IMO you will be disappointed with the AR 150's, not enough power. The 250's will do the job, but are a very pricey option. I have tried many amps with my Maggies and the best by a wide margin are the tube amps that I use now; the Carver Black Beauty 305's. The Black Beauty's use six KT-120's per side and seem effortless at driving the 20.7's. I never feel that I am lacking power or dynamics no matter how loudly I play music, rock or large scale classical. My room is failry large too, 17.5 x 26 x 9. The Black Beauty's make the 20's fill the room with sound. They will not be quite as extended on top as the AR but they will be more 'organic' with richer tone. You can tweak that a bit by changing the 12AX7 input tube on the amps, adjusting bias, and by using the built-in feedback swtich. Check out these two reviews and this video review on Utube:
http://www.theabsolutesound.com/articles/carver-black-beauty-305-power-amplifier/
http://www.enjoythemusic.com/superioraudio/equipment/0512/bob_carver_cherry_180_tube_monoblock_amplifier.htm
http://m.youtube.com/watch?v=uBBlDNCW0UE