I had to read your post a couple times to understand the question -- sometimes the fingers work faster than the brain when you're anxious
-- but I think I have the gist of it.
When you get your sub and tell your Denon to cross over at 80 Hz and setting your mains as small, it is true that your receiver won't have to work quite as hard to drive the M80s since it will be relieved of the power-hungry sub 80-Hz duty. This shouldn't have too large an effect on what volume level you have to set your Denon to in order to achieve your desired listening level, however.
Your understanding of power and resulting decibel level is flawed. Keep in mind this rule of thumb: every 3dB increase in sound level requires 2x as much power. So, assuming the dB levels of your volume control are accurate (e.g. every 1dB increment on the display corresponds to a 1dB increment in your room -- and this is hardly ever the case), every 3 clicks of the knob will double the power used. Essentially, this means, in theory, that you still have 50% headroom left at 3 clicks below max.
