Wow, this is a tough one. Speakers are always a good place to start, but the stock GM head units are tough to deal with. If your back speakers are just the 2 6x9's that is a great place to start. If you have the 6x9's and the 6.5 (round) speakers then the 6x9 are like mini subs and adding full range 6x9's will not do much.
Even though you don't have a Monsoon system here is a post I wrote that talks about The Bass Eq Problem with GM Head units.
Hope this helps, and for what it is worth I ended up gutting my 1998 GTP and starting from scratch. I'm trying desperately not to do the same thing with my GXP.
