Ahh.... yeah, Pioneer'* have a humongous issue with sound out of the speakers. Lots of times, it'* the fact that they have a horrible internal ground.
Just recently, I did a custom install (amp racks) on an '03 Trailblazer, had horrible sound issue'*. Ended up being the RCA outputs on the deck were either loose, or not properly shielded, causing all sorts of problems. A new deck solved the problem.
One thing you can try, when you got the radio out of the box, there should have been a white piece of plastic with a screw bolted to the back of the radio. Put a ground lead where that screw went into the heatsink, and ground out the chassis to the vehicle itself. Seems to have fixed a lot of the problems most of the Pioneer'* come into my bay with.
2000 Bonneville SE