Hi dazed,
Why can't you access the receiver menu and switch the sub to "Off" and the stereo left and right mains to "Large"?
Do not connect the A and B speaker outputs to your M3s at the same time. The receiver's amp for the main channels would then "see" the nominal impedance of the M3s (8 ohms) plus the impedance of the B speaker output (whatever circuitry/switching is there) and the extra speaker cables. Whatever interaction might result is not desirable. And I hesitate to predict what would happen to your amp's output section should you ever inadvertently have both switched on simultaneously. Nothing good.
Regards,
