Power consumption questions

When you plug an external multi-channel amp in to an audio-video receiver for increasing power output and overall performance, I'm assuming the power supply in the AVR shuts off automatically. Is that correct or does it stay on and work in conjunction with the new separate amp for driving the loudspeakers? Or does it get bypassed but stays on and used for powering the preamp processor?

If it does in fact get bypassed for driving the speakers, how could I then calculate how much of the rated power normally consumed by the AVR would be reduced (in watts) if just used as a preamp processor? I'm talking about Rotel equipment here and this information is not given in the owners manuals or specifications anywhere. In my case, the AVR has a rated power consumption of 450 watts and the multi-channel amp 800 watts.

The reason I'm asking this is that I only have a 15 amp 120VAC circuit available for my system and with the potential addition of this mulit-channel amp, I could be coming close to maxing out considering the other appliances that would be in use on that same circuit. Thanks in advance for the replies.