Given the information, I can just leave the receiver spkr settings to LARGE and let the main speakers do their best, then tinker with sub's crossover to make it complement the main speaker's lowest frequencies. If your main speakers have their lowest -3db points, say at, 40hz, i'd start to set the subwoofer's crossover over setting at that point and adjust up or down whichever would yield the best smoothness(integration) between the mains and the sub. I'd assume the sub has a lower -3db point that your mains.
Alternatively, I could set the receiver's crossover to 80hz. There are advantages here. The receiver need not supply the current to deliver frequencies below 80hz which can be substantial and instead provide the frequencies above it with more headroom. And if the sub is sufficiently powered, it should take care of the frequencies below 80hz where it is supposed to be better at. Then adjust the crossovers on the sub up or down to your taste.
