Hardly anything, M1.2 is pure Class A, M2.2 is a highly biased Class A/B , operates in Class A for the first 35 or so watts. You won't hear a difference between the two at normal listening volumes with most speakers. The M2.2 is the perfect match for his MLs, MBL, and some other panels but you can get away with the M1.2 with the A1. Pure Class A is the ultimate!
david
FWIW, i always leave my Gryphon on full-on Class A even though they have 2 other 'eco modes' which reduce the amount of Class A. Honestly, i dont hear too much difference between middle and full-on, but i definitely hear a difference between lo and medium/high. And this is with my Wilsons where i know that it should not matter at all given their efficiency.