Here's my understanding:

The 755 has two toroidal transformers. If Outlaw uses them like most amp-makers, 1 is for the surrounds and center, and the other is for the mains. (They try to balance the power delivery capability.)

Seeing as how the surrounds get more content than the rears, having the surrounds as the mains, and the rears as the surrounds might make the most sense.