Caution: Without know which unpowered subs you're intending to drive, it is very likely that the M1 may not be a good match for them and you could end up blowing the amp or the subs or both.
Since the Yamaha has subwoofer out. Simply take a RCA cable and connect the Sub Out from the Yamaha to the M1's L,R Inputs. The M1's Speaker connections on the to the unpowered subwoofers.
In the Yamaha run auto-setup so that it detects the new subs.
After auto setup is complete, go through and verify that all main speakers are set to "Small".