Assuming by the power rating mentioned it's the XPA-7, per the manual input sensitivity is only .8V so your avr should be extracting the full power range of the amp easily enough. May well be your expectations as it takes a doubling of amp power to gain just 3dB spl....
ps well thought so by just starting at the last page of the manual, but the last spec entry seems that's only the 2ch (gen 3) version, the 7ch version seems to be 1.5V, but as mentioned the pre-out voltage is likely fine up to 2V, so probably still not the pre-out level (even if a nominal rating of 1V per the manual).