Which Integrated for Magnepan MG-12?

I would recommend the Cambridge 840 with your MG-12. This unit with its high current power supply (class XD) will produce 200 watts at 4 ohms... plenty of drive for your MG-12.( your MG-12 are 4 ohm's loads).
Should you have a CD player with a balanced outs you can connect it directly to the XLR inputs on the 840..

Musical Fidelity? ...A5, A5.5, A308.

I have listened to my magnepan speakers using an acurus a-250, cambridge 840a, and now the krell 400xi. I love the current set-up and to me the krell has done the best job at driving the magnepans.

I've just gone through an extensive integrated amp hunt (including most of the models you've listed). I settled on an Exposure 3010s2. My speakers are a tough drive like your Maggies. You might want to add it to your list. I'd place it on par with the Creek Destiny, but with a more balanced presentation. The Destiny tends to accentuate the top and bottom end at the expense of the midrange. The Bel Canto uses a class D amp which gives a very different sound The others are not in the same league, in my opinion.

Maggies are NOT a 'tough load'.
Low sensitivity, to be sure, partially made up for by being dipoles. Low reactance, too, which kind of means you don't need hign damping factor amps, (read TUBES).

'd' amps sound good with them. The W4S has balanced inputs which go well with a balanced CD player....like my CA 840c.

I sort of liked the 400xi Krell with panels at a listening session, but the darn thing just got so HOT I couldn't figure out where I could put it where it wouldn't cook itself.

I settled on a 'd' solution which gave very satisfying sound, had balanced inputs and is not only very nuanced, but has great pace, as well.

Now that I think about it, I did try the Bel Canto S300, but I still feel the Krell outperforms it with the 1.6QR. The Krell never really gets hot in my system. I also use the balanced inputs, but with the Krell SACD standard. It works really well.

Over the weekend we listened to the Bel Canto S300iU and Arcam A38 (the latter with the Arcam CD17 player), but in neither case with Magnepan speakers. The Arcam was OK, but not great (we do like the clarity of the Arcam sound, but it seemed a bit thin). We liked the cleanness of the Bel Canto - my wife, who's a classical musician, described it as "objective"(is that a typical characteristic of digital amps?) - but then the audio rep switched to a Rouge Cronus Magnum and the fluidity of the sound really struck us. And we later heard the LSA Standard hybrid amp (paired with the Cambridge Audio 840C) and we were very impressed with the deep soundstage, presence and natural sound of the LSA, which frankly I had never heard of before. So I guess now we're adding tubes/hybrids to our list (and perhaps bumping up the budget by a few hundred dollars!); we plan to see how the Rogue and LSA sound with our MG-12s. We'll also try to check out some of the amps you all have suggested. We welcome your continued comments and suggestions.

I listened to the 400xi for maybe 20 to 30 minutes at the hi end of moderate levels....the Krell was sitting out in the open on its own stand...nothing near it at all, above or to the sides.
The power supply got way hot. I could put my hands on it, but it was NOT very comfortable. The amount of heat boiling off the amp I would rate as 'lots'.

If it sat with shelving over it and restrictions to the sides, I'm sure it would cook in no time.

The speakers? some Maggie 3.6s and the music was just rock....

I owned Maggie 1.2's that I listened to with a McIntosh integrated at 160wpc into 4 ohms.

I then did an in house test with a Krell S300I at 300 wpc into 4 ohms. I heard amazing improvement as the Maggie 1.2's came alive. They needed the watts to bring out their dynamic potential.

I traded in the 1.2's (I had them for about nine months) for new Maggie 1.7's and am still liking the Krell. For roughly $2000 new it is hard to beat. The problem is now justifying even more watts to obtain the last bit of dynamics out of the 1.7's (Or should I just upgrade to Maggie 3.7's and start over again!).

The amp will get warm and needs to be ventilated. The manufacturer wants at least two inches of clearance over the top of the amp and and at least one inch on each side. I also allow an inch underneath.

I was able to prove that the protective circuitry will shut the amp down at volume settings of 90 or higher if there is insufficient top and side side clearance.

Just wanted to let you all know how this worked out. Despite my initial determination to consider only an integrated amp, I got seduced by Bryston separates and ended up with the 4BSST2 power amp and a used BP-25 preamp (totally blowing the budget). Bryston, as I learned, is a common match with Maggies and for good reason - neutral, yet not without warmth, detailed and with real presence. Now I'm selecting interconnects and power cords; obviously this is still a work in progress (and perhaps always will be).

Rogue Cronus.