I have a 2012 PP with the absolute base stereo (no 8 inch subs in the doors). I wanted it that way - those subs just aren't that great, are difficult to replace and I don't like the look of those huge chrome rings.
Anyway, here's what I did (with great results).
I took a pair of old MB Quart separates I had laying around and mounted them in a pair of adapter plates I made (the material cost less than $3 at Home Depot):
Then, for the low end, I took an AudioControl LC2i line level converter and completely disconnected the rear speakers (I prefer the sound stage to be in front of me anyway), and supplied it to an old Kenwood Pro Series amp (PS-150) I had, set the low pass filter to about 90Hz and hooked it up to an MTX Thunder 8000 12" sub in a box I built to tuck into the corner in the trunk:
At first, there just wasn't enough midrange (it was all highs and only deep bass), so I took the door panels back off and added some "Wirez" sound deadening (self adhesive stuff) to the metal around the speaker mounts and some small pieces to any large flat sections of sheet metal on the inner panel (I didn't remove the plastic vapour barrier).
After that, the door speakers picked up the mid range just fine and meet up with the sub perfectly.
Now, it rocks - I'd put it up against the Shaker 1000 system any day, it looks bone stock (so as not to tempt potential thieves) and, since I already had most of the equipment, it all cost me less than $200.
So yes, you can keep the stock head unit. Aftermarket ones are nice (and can add quite a bit of connectivity options), but the stock one sounds fine, and with the Auxiliary input, it can play anything you plug into it.