Hi, Jeff. Has this recently stopped working, or did you just now connect the amp and you don't get sound?
Some things that come to mind if you recently set this up, none of which are intended to be patronizing:
1. Make sure that you connected the amp to the PRE OUT jacks (they have shading around them on the back panel) versus accidentally using the multi-channel input jacks (which are labelled the same).
2. Did you run the auto setup on the Onkyo? If not, try that.
3. Are the input switches on the back of the XPA-3 set to "Un-Bal"?
4. Are the status lights on the XPA-3 blue (if you have no lights, toggle the switch on the back panel to show them)?