My opinion is to start at the 80hz and lower to say 50hz and see which point works best in your room and setup. What does your room correction set your setup? Even with capable towers, letting the subs do their job does support more lower mid and upper midrange resolution. Also, using the 80hz suggestion will also allow most receivers filters to be applied to the subs. Example, my 3 standing wave issues are this: 63hz, 74hz, and 96hz. So, with my Elite receiver 2 of the three filters are applied to the sub signal. Or, when I cross the subs at 50hz then my towers which don't necessarily need as much correction due to output may not benefit as much. With todays LFE's and the huge dynamics with a smooth response, your room would be the deciding factor.
Brian in Bakersfield...