The general wisdom is to keep your Yammy tuned to 8 Ohms. I have a pair of QS8s, which are 6 ohm speakers, and I run them on a lesser powered Yamaha RX-750 without any problems sonically or from a heat dissapation point of view.

Hopefully others will jump in and tell you why. Also, run a quick search for this 8 vs. 6 ohm issue - I bet there's also some prepackaged accumulated knowledge just waiting for you to tap it.