Anytime that I have used 6 ohm speakers with an "8 ohm" receiver, I have never had any problems. Just because a speaker has a certain rated impedance, doesn't mean that it operates at that impedance at all frequencies. It's too complex to explain, but the impedance basically fluctuates with frequency. The impedance rating is an "average" rating. In your case, I would not pay attention to this issue. By the way, being a Yamaha owner myself, Yamahas don't care unless you try to run something rediculous like a 2 ohm load on the amplifier.
