My experience in this came from a 3rd gen Camaro. No matter what we did, it had no bass coming from the 6x9's mounted in the factory locations and baffles. We were told by the stereo shop that they were the reason it had no bass, and that nothing would change that. Just remember that the whole 6x9 adapter thing is far from ideal. They can sound OK, but they will never sound amazing. For that, I would suggest using a sub for bass and a crossover to keep the low frequencies from the other speakers. Even a single 10 in sub with a cheap amp will sound respectable if you are not trying to play booty music. Just be sure you are up to snuff with the satellites as they do far more for sound quality than subs do. You also want bigger front speakers than GM used in almost all of these cars. 3.5's just don't cut it for the front sound stage.
I'll list my system here for comparison: 2 Orion Cobalt (their cheap line) DVC 10's running a 2 ohm load into a Alpine V12 amp delivering a peak of 500 watts to the subs, Alpine CD player, Phoenix Gold separates with 5.25 in mid woofers in the doors and 1 in dome tweeters in the dash run off their own crossover network (installed using stock Regal Concert Sound II mounts and grilles), 2 Pioneer 6x9's in adapters (that I can't find the wire hanger bales for)...for now, and the satellites are all run off a 75 watt per channel Alpine amp. So, at best I have 800 watts and 8 speakers in the car. Most of the stuff is at least 10-15 years old, but still in good condition. The only new speakers are the fronts, and I got those on E-Bay for $65. The rest came from deals made with friends for their old stuff. I have a lot less than you would think in it, but it still sounds pretty good. I wish it had a better deck as this one lacks the built in EQ and sub gain/crossover that the JVC in my truck has. Plus, I could really use an iPod dock!