You could buy some 18 guage(lamp cord size) and splice them together. Any local hardware store would have the wire.
Interms of the lenght, it depends on how far, but for a computer speaker subwoofer, I don't think you will notice a difference in quality.
Also, sub frequency and sound is pretty much non-directional, so you can place it anywhere. For a cheap sub putting in the corner of a small room would be best. I'm using good a quality home subwoofers
so a computer sub might be different in sound.
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