Sure, it's commonly said that to be perceived as "twice as loud"(whatever that means)requires an increase of 10dB in sound level, which requires 10 times the power.
The power measurements are fine and exceed the rating numbers. As with most receivers, the Yamaha rating is in accord with the FTC power regulations which require two channels to be driven simultaneously at the full rated power for at least five minutes continuously. All channels driven is neither a legal nor a real-world home usage requirement for the power rating, since all the channels aren't called upon to output full power simultaneously on real program material. Note the comments of the tester on this point at the end of the second page in the linked 661 test results.
