Assuming your A/V receiver's line-level subwoofer output has a fixed crossover (low-pass filter) at about 80 Hz, you set the subwoofer's variable crossover at its highest setting (120 or 150 Hz, whatever the range is) to avoid the "cascading" of two crossovers. Then, if you need to, gradually adjust the frequency of the subwoofer's control downward to achieve a smooth blend between the sub and the mains with no audible "holes" in the bass. That's the ideal, but feel free to experiment with different subwoofer crossover settings. I certainly do with my subwoofer.
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