If you can't disable the subwoofer's crossover, set it as high as possible (120 Hz in your case, I suppose). Then set the receiver's crossover as appropriate for your speakers (80 Hz tends to be a good starting point).
I recommend that you set your fronts to small while using the subwoofer unless they're truly full range (meaning that they have a relatively flat response to as low a frequency as the subwoofer); if you enjoy hotter bass (especially for movies, many people do), just adjust the subwoofer's gain higher instead.
That is, unless some of your speakers only reproduce down to a relatively high frequency (say, 120 Hz) and your receiver doesn't provide any per-channel crossover settings, then setting a higher crossover frequency and the fronts to large at the receiver may help avoid audible localization of the subwoofer.