i replaced the syock head unit, i think within the first week, and am now on my second aftermarket unit.
i added a single 10" sub with a friggin huge amp, but left the stock speakers alone.
i am satisfied with the sound quality, although i will eventually replace the rear speakers

 

I totally agree needs a sub. My 99 sts came with a 450 watt bose system with a sub and it blows this one away, and paid an extra 4 beans for it

 

ya gotta ditch those factory speakers. They sound worse than blow Delcos, man. They only signed an agreement with Boston on these as a selling point. If you look at the magnets on em, you'll see anything at radioshack would be considered a step up.
I went with Boston Pro 6X9s in the back, Boston Pro 6.5s in the doors and Boston Pro tweeters in the dash, along with a 12" sub and 2000 watts to power it all.
Ya don't really have to go that far but at least swap out the speakers and you'll hear a huge difference. Don't blame the factory deck (although they are pretty cheezy too). It's your speakers that are bringin' ya down.
Bostom Premium Sound... wonder how much we paid for that little plastic thing in the corner of the window that has to be removed.

 

So I can replace the stocks with better Bostons or MB quarts and not upgrade the head unit or put in a AMP. I am thinking if I upgrade just the speakers and not anything else it will sound even worse because there is not enough power

 

i went with a whole upgrade. money well spent. never should have paid for the boston, but i had no choice.
i thought the boston system was pretty good until i installed my stereo. an extra 200 watts won't do it (imho).
running 2000+ watts and loving it.

 

well, lets' not get the crap the dealerships pushed on us confused with real Boston stuff. Two totally different things. Boston Acoustic doesn't even sell that crap to consumers. They'd go belly-up if they did. They just made this cheap stuff to give a good deal to companies like D/C.
My Boston Pros ROCK! Every boston acoustic product I have ever had (besides the stock stuff mentioned above) has been wonderful.