If you set up the subwoofer as a subwoofer, not a substitute woofer, then you'll still need the same amount of power flowing to the main loudspeakers. With a crossover at 80Hz the main loudspeakers will still have significant output at 40Hz.
Speaker relative efficiency/sensitivity when using a high pass filter
Is there a formula/algorithm/rule of thumb for calculating how a speaker's power requirements might change when the lower frequencies are handled by a subwoofer? Specifically for a ported speaker with a supposed frequency response down to 35Hz and an a 89dB sensitivity with a second order crossover at 80Hz. I know it goes down but by how much? I recognize that power requirements increase as frequency decreases but is the difference enough to allow for a less powerful amp? I would think so.