Has anyone added an external amp to their stock system? I was considering a 4 channel amp, in the 40 watt arena, to power six of my seven speakers, minus the factory sub. I like the stock system, but it sure would be nice to clean it up with a little punch. The stock amp is said to be 66 watts at 3% THD! I think that's 66 watts divided by six speakers, with a lot of distortion. If anyone has any suggestions pass them on.