Sometimes (don't qoute me on this)some companies rate the output based on 2 channels driven rather than all channels driven. Like for instance my Rotel is rated at 100w x 5 and it is. But for 2 channel stereo my system pushes about 128W x 2. Some companies would actually say my system pushes 128W per channel. But I know at least for Rotel, Denon and NAD and most high end separates they for sure tend to be on the conservative side. What devices they use to measure this . I don't know exactly what they are but here are some articles using some sort of device to measure this. ie. HCC measured output at onset of clipping HCC Fidelity Firewall
These are some articles that actually have some sort of devices to measure rated output vs real continous output on all channels which happen to be the model under yours.
http://www.homecinemachoice.com/testbench/frame.html?http://www.homecinemachoice.com/testbench/Amplifiers/Onkyo/OnkyoTX-DS787.shtml
Actually all receiver reviews on
http://www.homecinemachoice.com/index.html test for rated vs real continous power all channels
