Please help me understand amplifification

[amper]

I am getting a Yamaha Rx-880 to power my 5.1 system. I have Paradigm speakers, Monitor 7 series in the front and center, cinema series for the rear. I used to have an Anthem mrx receiver, which seemed to power them well at reference level. In another thread I was encouraged to get the 880 because it has pre-outs. I understand that although I am supposed to get 100 watts per channel with the Yamaha, I might only get 30 because it is processing too much information. So having a power amp for the fronts and center will have them perform at reference level the way they were intended to sound. (how much wattage do they actually need for reference sound?)
After my Yamaha purchase I dont have a ton of money in the budget, but I saw an Emotiva BasX A-500 Five Channel Power Amplifier at a decent price.
Question, if I don't power all 5 channels with the Emotiva Power Amp, as the rears can probably be powered fine through the Yamaha, will then more power go to the 3 channels I use? Also, since the front and center will be powered by an external amp, does that mean the rears will get more power from the Yamaha?
Am I making sense. I am new to this, as I used to just have my MRX do all the work just fine.