ot if he only wants to use one sub. Using a headphone splitter would require both subs be plugged in, since the speakers plug into the sub using component cables. The adapter above will allow the OP to plug all four speakers into one sub
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I was thinking about using a 15 pin splitter of some sort. One end connects to the controller and it splits to be able to plug into both subs. I am not sure if that would work though..