Well the kicker is the rear speakers dont make or break the car.
I dont have the monsoon system like you have... But when i put in a real system I removed the rear 6 x 9's. Im back to stock for about a year now without any rear speakers and nobody seems to notice or miss them one bit in my car...
So I wouldnt let the dumb 6 x 9's in the back throw you for a loop.
If you want to upgrade still...
Get a head unit first, run to stock fronts.
Get some speaker tweaks from
www.secondskinaudio.com and install these
Get a set of components 5.25" in your price range ($80-$10,000 is the range).
Get a proper amp to push purchased components and proper wiring.
Or buy it all at once if you got the $ saved.
If you do replace the system at all costs you are best served without the 6 x9's (or any rear speakers). Most new people cling to them like they are special, but they are the worst thing in any system period- (unless properly set up!)...