The coil is now included with kits, so nothing else is needed. If you're doing your own, just install a 0.5mH coil in series with the woofer. Cut one of the wires going to the woofer and splice the coil in, covering the leads with heat shrink tubing.The coil should use 20 guage or larger wire, which will result in less than 0.5 ohms DC resistance. A suitable part is the 260-723.This part has 0.5 ohms DC resistance, so the woofer's Qes is only raised by 0.03. The difference in overall system Q is small, so it works pretty well.