jvc - no subwoofer on line in

hey. i have a JVC cd player, its a KDLH series and i have problems with the subwoofer out. it is functional at all times except for when i use the line in input from the back. i have an mp3 player, so i hook that up and plug it in to the back of the deck, the line-in input. when i listen to music this way, the subwoofer recieves no signals. can anyone tell me why the sub wont function on line in input, or how i can make it work? thanks for your time.