You need a good amp to drive quality speakers, & with one, the stock HU sounds fine. If I didn't want to put a sub in the trunk, I'd get a 4 channel amp to drive just the speakers. And guess what? You don't need dual or even 12" or 10" subs to get good sound in the Mustang either. Best yet all the components cost me < $1000.
Really the general answer is rear speakers are not needed or called "rear fill" and are generally first to cut. Second the amps I liked from sub $200 range do the job just fine for running front speakers, rear speaker, and sub. Unless you want alot of power but then you need a nice mono block and separate amp for speakers, and at that point caps to regulate power.
Rear Fill.. No sounds stage for rear passengers??? - DIYMA Car Audio Forum
only good reason I think of for getting an expense Amp is you can generally pull an AMP out of a car, were the deck or the speakers I generally leave in space. To much of a pain to remove.
2011 GT Coupe Premium | MT6 | 3.73's | Security Pkg | BREMBO | Comfort
Installed JLT oil catch can, Tint, Beltronics RX45/shifters