send a signal? sound card? im not a professional in speakers guys, why you using fancy words and that?
The (audio) signal is the unamplified sound. Send it to an amp + speakers (or to powered speakers) and you have sound.
To get the (audio) signal out of the computer, you need a sound card. You currently have your speakers connected to one. If the sound card has a subwoofer output, you can connect a subwoofer as well. If it doesn't, you need a new sound card.