I am a recent GXP convert myself. My old GTP had a completely aftermarket system that I put in myself and was (am) VERY happy with. My GXP sounded like crap to me as well. I am currently in the throws of converting stuff.
This forum article was a big help: http://www.grandprixforums.net/facto...loc-32533.html
So far I have I found that one of the door tweeters had fallen apart and I think this was a big part of my problem. I replaced just the tweets with JBL GTO18t's. They are little too efficient however but much smoother than the OEM's. I have also added a couple of 10' subs and 400 watt amp. I tapped into the Monsoon Harness with a MTX RE-Q 3 using the speaker level sub signal going to subs in the trunk. Added caps to the existing subs (still playing with the value but I think I'm above 80Hz now maybe like 100Hz or so) Sounds pretty good, but not as good as my old GTP. I found that running the EQ levels on the head way down on the negative side works the best and is more like a "flat" response. This combined with the RE-Q seems to flatten things out in terms of the bass response. MY Monsoon system seems to have a high noise floor IMO. Next summer may see the whole Monsoon amp removed, a 4x line driver and all new speakers and speaker wiring all the way around. I used headphones with the line side of the system (before the amp) and it seems pretty clean and noise free, so my guess is that the Monsoon amp is putting out a lot of noise (what you hear when the volume is at zero).
Big problem is I like the integration of the head unit (HUD and stuff), the steering controls are and have been a non issue for some time, I did that in my 1998, and it has worked great for many years with a Sony head unit. There is also a PAC's unit for the chimes, On-Star and such.
Interested in your thoughts.
