I have found a solution for the sloppy rear sound of the rear speakers. Of course, this is BLASPHEMY because its not the stock, GM-blessed-by-the-hand-Zora "look" ..... but it serves its purpose well.
Remove the factory speaker and assembly (if the previous owner hasn't done so already). Now, purchase a 6x9 speaker enclosure and remove the the bottom corner as picured in the attachment- it wil fit perfectly over the factory speaker location and only protrude four inches out. You want to have the wires enter through the side that butts up to the carpet so as to conceal the wires. You can drill through the enslosure and use the original speaker mounts (threaded self-retaining clips). The sound quality is phenominal compared to the original. Additionally, it will give you the clarity provided by the enclosure without mounting any clunky floor mounted boxes.