The AVR is not designed to power that many Zone 2 speakers. Suggest you purchase a dedicated 2CH amp (see one below) and connect to the Zone 2 pre-outs to power the speakers.
http://www.amazon.com/AudioSource-AMP-100-Stereo-Power-Amplifier/dp/B00026BQJ6
http://www.amazon.com/AudioSource-AMP-100-Stereo-Power-Amplifier/dp/B00026BQJ6