I can't remember what receiver, but some model had a warning against hooking up speakers when the pre-outs were connected to amps.
I do remember that someone used a Kill-A-Watt on a previous Pioneer Elite flagship receiver. He found that when there were no speakers connected to the internal amps they still drew some power vs. going into the settings and disabling the amps. It wasn't much, like 5 Watts, but that does add up over time. My current Pioneer, while having pre-outs has no setting to disable the internal amps.