Best bet with monitors like that is to get a subwoofer with RCA inputs and outputs. Then you run your music source to the subwoofer, then connect the monitors to the subwoofer. Better would be to get one that also has a high pass filter on the RCA outputs so you can crossover the sub with the speakers at a higher point than the natural rolloff of the speakers. For example, this Velodyne DLS 3500R
allows you to choose from an 80hz or 100hz high pass filter, then you set the crossover dial on the sub to match.