The backpanel power rating is NOT a max power rating unless it states it. It's typically 2 channels driven at full rated power for 2CH amps/receivers so 140wpc x 2 + say 50 watts for processing / 0.5 eff = 660 watts but for multi channel receivers they typically run 1 channnel at rated power and the others at 1/8th power which is much closer to the 500 watt rating on the backpanel. This is how UL does their heat dissipation tests