Fred,
here again are the regs for reference. You'll note that the title relates to "home entertainment products", but so-called "pro" amplification equipment which is also sometimes sold for home use must also be in compliance.
The quoted Peavey power specs do in fact comply with the regs, stating power, impedance, frequency and distortion. As required by interpretation, this is with both channels driven.
If I follow what you mean by "translate", yes, the consumption figure(and of course it's consumption, not output)can be viewed as each channel being involved in the consumption of 580(not 560)watts. The indication from the numbers is that since the 1/3rd power figure is 533 watts({2x800}/3), the efficiency at that output is about 533/1160=45.9%.
