AVS Forum has a nice thread dedicated to the Marantz SR8400 Receiver and Marantz SR8500 Receiver. The linked page contains information which is either not included or that I was otherwise unable to locate in the manual for this receiver (and I've read every word of that forsaken thing) -- how to change the A and B speaker selectors via the universal remote control that comes with the unit.

Press AMP and then the 6 button.

Now, if only I could find the original universal remote control...