Lets say your Denon does 300 watts per channel at 8ohms. Split that value by 3 and you should get closer to 100 watts per channel for each of your LCR speakers. I am winging it with the 300 watts. It could be +/- 10 to 15 watts (usually minus) depending on source material.
Ideally; you want to do the reverse of what you want to attempt. The power hogs are the LCR's. The ceiling (or atmos) and rear surrounds are generally okay to run off the AVR amplifier.
You can do what you want to do but ideally, you want that power amp to push nice clean power to the more demanding channels, which are the LCR's. The spikes on the effects channels are a lot less demanding unless there is armageddon taking place on screen. Its not continuous.