best amp for Magnepan 3,7

For brands, I like Pass's XA.5 series on my 3.6's.

I side with Ahendler on the power issue, it just depends on how loud you like to play your music and the size of your room.

Agree with Onemug, size depends on how loud you like to play them and how close you sit. In general, if you like to play the loudest orchestral music at levels approaching natural ones, e.g., 115 dB on the peaks (which is maybe 100 dB on the typical SPL meter), you will either need a big amp, or to accept some clipping. But many or even most people never listen that loud. If you back off 10 dB, you've reduced power requirements by a factor of 10, but perceived loudness by only 1/2. So the only real rule on size is to get what's right for you.

I'm running my 3.7s with a VAC 300.1a (150 tube wpc). My room is 18x25 with pitched ceilings. No clipping, no distortion, no running out of steam -- tight present bass too. I don't like to listen at volumes that damage my hearing though. Another option is hybrid tube amp like the Moscode 402.

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.