For starters, you will need to use a powered subwoofer with analog inputs (either RCA or 3.5mm). Simply connect the subwoofer to the RCA outputs found on the back of the HD6 speakers. If your subwoofer only has a 3.5mm input, you could pick up a 3.5mm to 2-Male RCA Adapter cable to bridge the connection between your subwoofer and the HD6 speakers.
Please refer to the image below, using our S8 as a reference:
Please refer to the image below, using our S8 as a reference: