Using Preamp Main Outputs?

I recently purchased a pair of main front speakers that are less efficient by several dBs than my previous main speakers. When I crank up the volume to the main speakers (relative to the other channels) it seems like the receiver does not have adequate reserve power to drive the system properly.

My Sherwood 5.1 channel HT receiver has preamp outputs for all channels and I have a nice powerful MOSFET amp that I'd like to use to power my main speakers.

1) Can I use the receiver speaker outputs to drive the center and rear channels at the same time I use the front main preamp outputs to drive the more powerful amp and main speakers?

2) Will the receiver somehow sense this hookup and stop feeding power to the main speaker amps (as there will be no speakers or load hooked up)?

3) will the small speaker vs large speaker settings (and the individual channel volume adjustments) on the receiver still work the same?

I'm sure this is very basic stuff; I've just never had the need to try it until now. Thanks for any advice!