J.B., of course that's true, since as was pointed out above, the receiver connection to the mains runs straight through the sub to the mains, essentially the same electrically as if the wires ran directly from the receiver terminals to the main speaker terminals. There's no mechanism by which any power could be "lost".
There's in effect an internal "Y" inside the sub with one branch going into the sub amp and the other going straight to the high-level outputs. The input impedance of the sub amp is on the order of tens of thousands of ohms and practically zero power can get through that.
