You are confusing the low end of the frequency response with crossover frequency. Just because the speakers are rated to extend to 75Hz and 55Hz doesn't mean you should set the crossover to those frequencies. I would cross them another half octave or so higher which is ~85Hz and 110Hz so setting them both to 100Hz should be fine. The risk of having them a bit higher than "ideal" (whatever that is) is that some low bass sounds may have their imaging pull toward the sub more than you would want. The good part about crossing them higher is that you put less of a load on your receiver.
If, on the other hand, you had some monsters that were rated to extend down to 30Hz or under, you would truly be wasting that performance by crossing them at 100Hz.
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