From what I remember with my 845, Sony recommends that you don't use A and B speakers at the same time if either pair is 4 ohms. For A + B, it says use speakers between 8 and 16 ohms. For A or B, use speakers between 4 and 16 ohms. I might be mistaken, but I know my 845 couldn't handle too much of a load (especially at higher volumes). However, it would be fine if you weren't running both zones at the same time.
Jason
Jason