<font color='#0000FF'>I don't know about the Onkyo, but these might help:
1. Set the proper distances from the receiver, this is the first thing. Use imperial (feet) not metric if you have this option. If you use meters, the case in my Sony STR-777ES, you will be adjusting the distance in 1 meter steps, which is in 3.28028 feet steps, while if you use imperial you would be setting it in increments of 1 foot, which is .3048 meters, more accurate.
2. Set the rear speakers to small, so that you would protect them from low frequencies, which will harm your speakers on high volumes, in your case, your rear speakers are going to play louder than normal.
3. Use the test tones to adjust the level of your speakers to be equal from your listening position. A Sound Level Meter might be helpful in this regard.
I'm not an expert, but these are the basics that I know,
Good luck,</font>