I do not think you need a pre-amp. Seeing that you are in santa monica, i'd recommend Audio2000 on olympic blvd. just west of the 405. They have good rates, and my friend who I recommended to them was very happy as well. They always have a few nice cars there that they are working on. I'd definitely check them out. I'd also check out how your sub is wired up. Depending on what type voice coil your w6 has, you might be only pulling a small wattage out of your amp and need to wire it for a lower resistance load. (If I remember right, the w6 is a dual 4ohm voice coil, so you have many options of what impedance to run)
http://www.audio2000.net/