Disclaimer: I am an audio dealer who sells some of these products. I had a set of MG IIIa's in the store a few weeks back and played around with some of our store amplifiers.
The Pass X350 on up and Wolcott P220M's are the two best I have heard. The ARC vt100 is quite good too but can run out of power. For cheap the little EL-34 based Conrad Johnson MV's are very very good - just don't expect enormous power.
The surprise hit is the Quad 909 amplifier. Sweet with tons of power. Absolutely superb match (it's pretty good with Quad 988's too !)
In general and in the past I have found that Magnepans typically sound their best with tubes. The trouble is that they eat tubes like there is no tomorrow. There are a few solid state amps I have heard as a good match but that does not seem to be the general case. Sometimes the Maggies can sound transistory or lifeless with these. The first generation Krell KAV amplfiers fall into this camp (the new KAV stuff is supposed to be outstanding).
