If you look at the line in/optical socket on the jb3, you will notice that it is recessed into the side of the player, and that along the top side the top piece of the player sticks out. Some wider 1/8" plugs will not fit in all the way because they hit the top part which sticks out. You have to shave it down, or find a smaller cable.
If you look in the creative usenet group there are pics.
JAson