That depends on how loud you are pushing the system, what the actual bass extension and dynamic output capabilities are for the loudspeakers and how good the subwoofer is. At some frequency point the subwoofer will simply have higher dynamic output and lower distortion and becomes the better choice for handling the redirected bass.
Also, most loudspeaker F3 ratings are exaggerated to some extent and you may want to use REW and a UMIK-1 mic to determine the true in-room extension of the speakers.
I would encourage you to try both 40 Hz and 60 Hz and see which you feel performs better. I'll typically recommend 60 Hz even for bass-capable towers, because of the aforementioned HPF slope being fairly shallow. Even with a 60 Hz crossover, the mains are going to handle plenty of bass into the upper 40 Hz range before the filter has an obvious effect on output.