The OEM head unit is the cause of your crappy sound. Your sound is only as good as the source. You will be guaranteed distortion using the stock H/U. Many people think upgrading the speakers will solve the problem, it usually just masks the problem.
You can take a system with a good headunit, stock speakers, and an aftermarket sub and make it sound 10x better than a OEM headunit with upgraded speakers and an amp running off a LOC.
My suggestion is upgrade the headunit, add your sub. With proper set-up will be what your looking for.
FYI...I professionally installed car audio for 8 years, so I'm not just spouting off "I can read a magazine" opinion.
2009 Night train