i'm new hear to the GTO scene, I just came from the Scion Scene, I turned in my 05' Sooped up Xb for my goat.
I know hard core stereo's and also work for Fujitsu Who makes Eclipse Audio components. If you want the big boom without sacrificing minimal trunk space in the GTO, go Eclipse, they have the least Box surface requirments on the market. You can get a compact box inserted to the side of your trunk with one SW9120 or a SW9100. It will rump harder than any blaupunkt or any other dual sub set up out there. Just upgrade your other component speakers and definately your head unit for best sound quality and proper channeling. The speakers that come with the GTO are clearly teribble, and respectally shows how "Good" blaupunkt really is.
Anyway's, if anyone one lives in my area (Laguna Hills, CA - SowCow) and ever needs car stereo install help, i'm more that willing to volunteer to help out, that is if you provide lunch! LOL. I've installed custom stereo's for some years now, I know that Eclipse is at the top of the game when it comes to ultime sound quality.
I trust some will find this info helpfull, and for those whome already know car stereo lingo, respects... Enjoy.