Viv, your 3803 has the option of a 40, 60, 80, 100 or 120hz crossover setting. Theory says to set it in consideration of the speaker with the least bass extension, so you might think that calls for 100hz, but those speakers will have enough left so that 80hz is the better choice. Don't worry about the M60s; it's better for the them if the sub takes some of the bass load off them. Nothing is being "wasted".

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Enjoy the music, not the equipment.