My shop is recommending I add 2 JL Audio 6W0's in a sealed box in the left back panel (where the nav/changer area is). Another post had said a BA 8 works well there, but BA no longer sells it. They also recommend I get a 4*75 watt amp, bridge the amp to run 150 total to the subs, and leave 75 * 2 for the front stage. I think I will go with an AVI set I have for the front stage. What have people done with the center channel? The shop recommends one or two smal tweeters in that location. Thoughts on system (keeping original HU) for now.)

While the sound might be a little better with an aftermarket head unit, an aftermarket unit will never look as well integrated as the factory HU. Changing the HU will also mean losing the info display for station/track in the middle of your dash. If you're trying to win competitions where the system will be played with the car stationary and the engine off then definetly go aftermarket. If you're just planning on listening while driving the stock HU is fine. Remember that the music will be competing with engine, wind, and tire noise, which in my 04 S4 Avant is a considerable noise floor.