Mark, welcome. The crossover setting itself is fine. Do you mean "kicks in" literally in the sense that the sub is only operating intermittently? Have you made sure that the sub cable is tight? Possible defect in the sub amp? The idea is not to have the sub set so loud that it's obvious as a separate source; the impression should be that it isn't working but that the M2s are putting out more bass. Of course, if the sub is actually not working, something's wrong.

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Enjoy the music, not the equipment.