best amp for Magnepan 3,7

More power is always better in my experience, my top choices for your Maggies:

Pass Labs X-600 (.5 if you can)

Pass Labs X-350.5

Sanders Magtech

Cary Audio CAD 500MB's (what I use with my 3.6Rs)

Bryston 7sst2

Parasound Halo JC-2

Wyred4Sound 1000's

I've had all these hooked up to my Maggies (with the exception of the X-600's, and the Sanders, but I had an older Sanders and give the new one some love for that). I have had a few other amps on them as well, but I wouldn't put them on the list.

McIntosh mc2301's. I have tried many amps, it was immediately clear that the big tube amps were a perfect match.

System synergy and room characteristics will influence your results to such a large extent that only an in home audition of the amp(s) under consideration will be of any value.

Expand the options and consider biamping with an outboard xover, tubes on top, SS on the bass, and xover a sub at 40Hz, say a pair of JL audio subs. A big room can now be filled.

The Halo JC-2 is a line stage. Did you intend to recommend the JC-1s?