Looking at an upgrade to mine. I bought mine with standard sound, knowing that I would be doing this...
Likely install after talking to Northfield will be:
3-way DLS speakers - 6.5" bass units (8" if they can squeeze them in, they will see when they do it) to replace the horrible horrible horrible bass boomy crap things that they call speakers under the seats; 2.5" midranges that will go in the existing front door spots; and new tweeters in the front door / window corner panel. Rear mids stay as they are.
Driven by an ArcAudio ks125.4 mini, bridged to two channels for the fronts. Rear channels (i.e. the mids in the rear doors) remain driven by the head unit's rear channel. No separate sub, but plenty of room to put one in later if needed.