Hook the speakers up to the speaker outputs of the AVR. If you have sound, then the sources and AVR are OK, and the power amplifier is likely the problem.
If still no sound, then there is likely a problem with the AVR. If both your sources (PS3 and Duet) are connected via their digital outputs to digital inputs on the AVR, the next thing to try would be to use the analog output from of one of them into either processor-bypassed direct inputs on the AVR, or switching the AVR to an analog bypass mode.
If that gives you sound, then it sounds like the Digital section of the AVR has gone bad. Or perhaps there is some kind of factory reset you could perform on the AVR to clear a jumbled processor state (not too likely, but a possibility).
Steve