which crossover system should I buy?

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My studio monitor is an Alesis M1Active 320 USB. My subwoofer is a Polk Audio PSW111. I could hear the subwoofer but very subtle even at max volume (I'd have to put my ears next to it, it's that subtle!). I have the right subwoofer cables I think but why is this not syncing with the Alesis studio monitors? Is it because it's not compatible with each other? Do I need a subwoofer receiver for it to be compatible? Which system should I look into?