Powered subwoofers used in home theatre systems use a very specific type of cable for connection to the home theatre receiver. This cable carries a mono signal from the receiver to the amplifier, and consists of a two-conductor wire with a single RCA plug on either end of the wire. If you'd like to make your own cable, either to save some money or to make a custom-length cable, you can do it with some basic materials and simple tools.
Cut a length of wire long enough for your needs by taking into account the distance between your receiver and powered subwoofer.
Strip the insulation from the ends of the wire so that each lead has 3/8" to 1/2" of bare wire exposed.
Unscrew the shell (jacket) of your RCA plugs. Slide them onto the wire so you can attach them to the jacks once they're connected.
Solder the positive lead of your wire to the tip of the RCA plug. Solder the negative lead of your wire to the sleeve of the plug. When the solder cools, slide the shell over the plug and screw it down tight.
Repeat Step 4 with the other end of your wire. When you're done, you'll have a subwoofer cable, ready to connect and use.