My room is nearly the same size as the OPs, but a different shape. I own a Belles amp (150A "Hot Rod"), and have Magnepan 1.7i in my main system. The Belles is in the bedroom.
I can't recommend any integrateds but I'm sure the biggest Hegel you can get would drive them. What I like, you may not.
I've run the Maggies with as little as 25 watts of tube power but IMHO they like big SS power. My current setup has monos rated at 850 watts into 2 ohms.
I don't find the Belles hard or grainy, but neutral/mellow. It would probably run out of steam driving the Maggies based on its performance with some inefficient bookshelf speakers.
If you can, audition the new amp in your room (nice room BTW). I wouldn't recommend a brand since we all hear something different. Just get good, clean, power rated down to 4 ohms, or lower.