sir hunnyko,
there will be no ill effect when pairing a 6ohms speaker to an 8ohms amp/avr, vice versa.
actually it is within the safe zone, i dont know about the other yamaha models, but mine has this switch at the back for ohms selection, and in one of the switches there is printed (6ohms-8ohms), what im trying to drive at sir is that the 6 or 8 ohms rating is not really a mandatory requirement as this equipment are expected to handle, 6, 8, 16 ohms load (amp/avr), while speakers though rated at 6 or 8 ohms ideally operates within the 6 to 8 ohms zone.
a word of caution however, there are power hungry speakers out there with ratings of 4ohm or lower, with this type of speakers, you need to have amps/avrs which specifically are rated to handle 4ohms load or lower, otherwise you will run the risk of over loading the amp/avr.
hope i have been of assistance.