After years of replacing decks, speakers and adding subs to previous cars with dubious, albeit loud, results I have finally laid that nonsense to rest.
The Sony 10-speaker setup that came with my SEL sounds amazing to me; much better than anything that I could piece together. In fact, it sounds better than my girls' stereo in her R32 and easily outclasses my co-worker's A-4.
It's crisp, clear and loud enough for me. Plus the proprietary design of the stereo/dash in most new cars these days does not make modification easy, nor necessary IMO.