In general, it is not advisable to select a frequency higher than 70 Hz. because if you do then you will hear where the sub is situated.
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You may want to check Harman's web site as there are a few technical articles by Floyd O'Tools team discussing the subject. I recall Richard Vandersteem talking about how a subwoofer needs crossing over an octave above the main speakers -6db lower limit, but I'm not sure about that. Hopefully this will get you started.