Hi John,
Just had this reply from Yamaha.com
maybe this will interest you.....
kind regards.Hello John:
In response to your email: Yes, all Yamaha receivers are compatible
with 4, 6, and 8 ohm speakers. If you are going to conneect 4 ohm
speakers to the A terminals, do not connect anyother speakers to the B
terminals or use the A and B speakers simutaneously. By doing this this
will drop the impedance down below 4 ohms making the speakers connected
to A and B not compatible with the receiver. Thank you! af