The best solution from ARC, by far, is the REF 75...try it, you'll like it.
I seem to recall that someone reviewed the Ref 150 and said it was adequate to drive the 3.7s, but implied barely so. I realize that the Ref 250s cost substantially more than the Ref 150, but I would err on the side of more power than less. Call Kal at ARC. He is very familiar with the Maggies and should provide excellent advice.
If you don't mind looking beyond ARC, consider Rogue m180's. I use the M120's with my MGIIIa's, tweaked out of course, with stellar results. The sound even better....by a wide margin, over the Classe I was using and the Classe sounded very good.
Sorry that I don't have experience with ARC but I would think that if you can swing more power go for it.
If you can afford the Ref 250 mono blocks and have the space for them that's what I'd recommend. I think the mono blocks would most likely sound better (by a slim margin) then the 150 but it's a matter of dimensioning returns and only you know your financial situation. I believe the Ref 150 would work nicely. I'm not sure if the Ref 75 really has the power you are looking for to drive the Maggies? As was mentioned Kal at ARC would most likely know.
Please post your findings here, because it's a really good question that has come up before but no one seems to post their findings.
I drive my 3.7's with a Plinius Reference SS amp so I think I have 450 watts per channel.
Don't know if this will help in your decision, but I have both a Sanders Sound Systems Magtech amp, which is SS, and a pair of VTL MB-250 Signature monos. I use the VTLs in the cooler months only because my room is above the garage and gets too warm in the summer. Speakers are Maggie 1.7s and I prefer the sound with the VTLs slightly over the Magtech because it has more dimensionality. A somewhat more spacious sound. The difference in power output between these amps is substantial, 900 for the Magtech into 4 ohms vs. 250 tube watts for the VTLs. The room is not a perfect rectangle but the approximate size is 19 x 15 with an 8 foot ceiling. BTW I just had to spend $4400 for a new furnace so something has to go and I've decided it's the VTLs since I can't use them all year long. Let me know if you are interested.
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: