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Crossover frequency depends upon not just one speaker set, but also on the other speaker or speaker set that is nearest to it in frequency range.
*Minimum* crossover frequency, for really subwoofer-type sound production, would depend upon how much wattage you are trying to get the subs to produce.
Overdriving the wattage of the speakers, or the subwoofers, will reduce their lifetime for sure.
If the S-107s were from a Sansui S3500, then the ratings for wattage output of the S3500 might indicate the wattage ratings for the S-107s. Does that sound reasonable?
I do not have that specific information, and must apologize for my having disappointed you.
To make things fair, I have Opted Out of this case, opening it to the Expert in this category who might have that specific information on the speaker horn's specifications.