Setting up 2 power amplifiers to setup at once help?

"So using the sub-out signal to drive the low frequency amplifier would cause frequencies between approximately 80 Hz and 125 Hz to be under-emphasized or even almost absent"

Good point, which is what I was trying to emphasize here-
"I can see a situation where the signal sent from the receiver is too 'low' for the difference between the frequency ranges of the low to mid speakers, which is why I advised to dial it up till it sounds full."

This is exactly why I didn't try it with my system since my stereo  crossovers are pre-selected at 60, 80 and 100 Hz and my speaker crossover is set to 125. I had thought an AVR would have a more flexible range. My mistake. 

Is it possible you use an integrated preamplifier with a power amplifier. In my case an integrated preamplifier with any of my denon or Yamaha power amplifiers ?

I've never heard of an integrated preamplifier though there certainly can be such a thing. If you mean an integrated amplifer then it depends some have the pre outs to connect external power amps.