You will probably need to connect your source to the subwoofer, and then use the line level output on the subwoofer to go to your powered speakers. This means that you need a subwoofer with the appropriate connections. And if the subwoofer does not filter its line level output, you will be sending a full signal to the powered speakers.
This also means that the volume control on the powered speakers will affect the balance between those speakers and the subwoofer; that is, the volume control on the powered speakers will not affect the volume of the subwoofer, only the speakers. You will need to use the volume control on your source (presumably, it is your computer?).