Your XP-20 is connected to front right and left speakers therefore the same channels, F R & L preamp outputs, should be run from the AVP to the pass through input of your XP-20. The XP in this instance only routes the signal from the AVP to your X 350.5. That way the processing, if any, and volume control is carried out by the AVP. I have a similar setup. I built a two channel using Classe CP 800. A couple of years ago I added a budget theater system to the mix and connected it in the same manner to the pass through on the CP-800, works great.
Mark
Mark