As I recall, it switches from class AB to class G around the 80W mark. I believe that this is done to minimize heat gain (allowing them to be stacked on top of each other without overheating) and cabinet size. The interaction and transition is described in
this post by Scott. Here's the most relevent part of that post:
The Model 200 utilizes a newly designed (and proprietary), hybrid Class A/B/Class G circuit. It will provide class A/B power up to 80 watts (which covers about 98 % of most listening situations). Above 80 watts the Model 200 will instantly shift into Class G amplification. This approach was chosen because of our requirements that this amplifier had to be quite compact (1 3/4 inches high) In addition we insisted that the amplifier would not use any fans. For those of you who are curious, the transition from class A/B to class G requires just two microseconds (NOT milliseconds),and (we know this question is coming) the transition itself is 100% inaudible. This was key to us in that some other alternative designs exhibit audible side effects. Incidentally at 200 watts the 200 has less than .05 % THD.