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!
1) Yes
2) Yes. Some receivers do this differently. I had a Yamaha DSP-2070 and when you plugged in a I/C to the pre main out, it disabled the internal amp. My Yamaha DSP-A1 had a shorting plug that you had to remove to plug into the pre main out. When you removed the plug, it disabled the internal amp. The best thing to do is read the manual for that receiver. It should tell you in there.
3) yes
Thanks Wccoe, I'll have to try it and see if it works as intended. It should. Now I just need to find a spot to shoehorn that big amp into the system. :)