Yamaha receiver power
I have a similar Yamaha (HTR-5860) and I am driving a pair of 4 ohm Onix Refrence 1's without any problems.  I recommend keeping the receiver on the 8 ohm setting so you don't limit the power to the speakers.  If you have 5-7 4-6 ohm speakers, then make sure you check the temp on the receiver.  If it is too hot to touch, then you need to turn down the volume or add a fan for circulation.  The receiver has pre-amp outs so you could always add a Behringer A500 or other amp for the mains.