Let's be careful here what we're referring to. When I said the factory head unit was "better bass" - that's literally all I meant - fuller bass with the stock speakers. As for overall sound quality, I agree that aftermarket decks always win. However, the factory head unit runs all frequencies through the speakers, and along the way, it gives a slightly fuller sound than the aftermarket decks I've listened to (with an otherwise stock setup) due to the bass. That said, I've also found the stock head unit will distort at much lower volumes than aftermarket units, and I have to start turning the bass down if I want to increase the volume any more without encountering distortion (I think it's a power saturation issue).
A lot of people without much experience with audio systems perceive "sound quality" as a fullness or even loudness, and usually make a judgment based on the bass (even if it's distorted). [side note - why do you think Bose sells as many of those little $500 clock radios as they do?] On the flip side, most audiophiles actually listen for a lack of distortion, and will gladly tolerate reduced bass for better sound quality in the rest of the spectrum. So even the word "better" means different things to different people.
There are certainly some aftermarket head units that have real amplifiers built into them, and they ARE able to create better bass than the stock head unit; but they are the minority. Most aftermarket head units are grossly overrated on their power output, and I think they cover that by limiting the amount of power they put out in the lower frequencies; it keeps them distortion-free farther up the volume range.
With any speaker, the fewer frequencies it has to create, the better a job it will do at producing the ones it does. This is part of why aftermarket decks have better sound quality - they are able to block the lower frequencies from reaching the stock speakers. Even the act of putting a subwoofer in and then putting a high-pass filter on the stock speakers will significantly improve their sound quality, and thus the overall sound quality of the whole system.
That's my two cents.
[ 04-23-2007, 10:21 AM: Message edited by: sherlock9c1 ]