An 80 Hz crossover is fine even for floorstanding mains (60 Hz would work as well also, but most recievers don't have that choice). If you were to actually measure the output in your room of your mains, then you might be suprised at the rolloff at lower frequencies. My mains are speced down to 22 Hz, but they really roll off at about 50 Hz.
With an 80 Hz crossover I am not using a little of my mains ability, but the Sub can do those frequencies even better, and the reciever will not be wasting power on the lower frequencies, will have more headroom, will be less likely to clip, and will run cooler etc. The main speakers will sound better because they are not straining to reproduce the lowest notes at the same time the woofers are producing 2k Hz notes (or whatever the crossover to the tweeter is). Some people even say they can hear the doppler effect when a woofer is moving out quite a bit for low bass at the same time as it is vibrating rapidly for the upper midrange. Reducing the max excursions reduces this effect.
Also remember that the crossover is not a brick wall, but is gradual. There is lower information still going to your mains.