I decided to post this here as there are so few current upgrades for the Bonneville GXP'*.
I got tired of the thumping of the Monsoon system, especially on any music with a decent bassline. Figure an 8-SPEAKER MONSOON SOUND SYSTEM should have good lows. Well, the problem isn't the system but of course the crappy speakers. I went to http://www.crutchfield.com/ and found a set of Polk Audio db 690'* for the rear and Polk Audio db 675'* for the front door speakers. The installation is pretty painless, although believe it or not the rear seat comes out for the back speakers :( It'* almost like they built the car around the speakers themselves. But Crutchfield tech support was able to give me detailed instructions for removing it. They also have photos for removing the front door panel as there is a tricky hidden clip in the center of the door.
Anyways, the sound is AWESOME compared to the OEM setup and the factory grills remain. Crutchfield also has no hassle 30 day return with return shipping! You can't beat that! They also provided free a GM wire harness as GM finds it so difficult to use regular wire plugs, sheesh! Right now Crutchfield has a buy one set get the second half price until the end of August which saves $50 on the front speakers. And if you mention referral code pg6xf-iycua-avb2z they'll knock off another $20.
I'm sure anyone who takes advantage of this sale will really enjoy it. The Monsoon amp is pretty good, it'* just a shame they stuck such lamo speakers in the car with it. BTW, I only did the 4 speakers but was thinking maybe I'll get a set for the rear door also.

Thanks for the information. I have a question concerning the ohm rating of the speakers. When I had my '02 T/A convertible, the Monsoon system in that car used 2 ohm speakers instead of the usual 4 ohm. It was very hard to find 2 ohm replacement speakers. Is the Bonneville different, and uses 4 ohm speakers, or did you have to crank up the volume with the new speakers to have the same level as the old 2 ohm speakers? Just curious...

The old speakers are all 4 ohm but the sensitivity must have been lower then the Polk'* as the new speakers sound much louder at a given volume. Could be the 3 way system as opposed to the single speaker.

Just ordered the Polk db525 for the rear doors. Crutchfield recommended a bass filter as they said the speakers would start distorting well before the front door/rear deck speakers. If I sit in the back seat with the radio playing I can definately hear the OEM speaker clipping so I think we're on the right track.
WOW, can't wait. It'* like a whole new system. Next might be a powered subwoofer for the trunk.

I ordered the Polk'* for all 3 pairs today. Did you do your yourself or ??? I don't know what all is inside the door panels and don't want to risk messing anything up.

I did the Polk'* on all 6 speakers. The biggest difference is more highs!!! I like it because the rear speakers were so muffled sounding! Can't really tell any difference in the loudness but definately a lot clearer.