Does the JL sub have dual voice coils? Each voice coil is 4 ohms. Since you hooked them up in parallel, the total impedance is 2 ohms to the amp. You have to hook the two voice coils in series. Amp pos to vc1 pos, vc1 neg to vc2 pos, vc2 neg to amp neg. That will give you 8 ohms to the amp.