First upgrade should be your in door speakers. Transducers are the lifeblood of any sound system, and the answer to "which amp to drive them with?" should always be, "The one which matches the power requirements of the speakers." Modern amplifiers are all fundamentally the same until driven to their limits ( I.e. output stage clipping our power supply current limiting). That being said I find the stock system totally acceptable for what it is, which is a SYSTEM designed to work as such.
The settings that I have selected on the non-nav head unit are:
Bass +2
Mid -1
High +1
I am a professional audio engineer who has mixed live concert sound for many major, international music acts. I am gainfully employed and well compensated for my critical listening abilities. I've worked in recording studios as a staff engineer, am certified on many professional live sound speaker systems, and have an in depth knowledge of sound system tuning.
If people are interested, I'd be happy to take some in car measurements with my dual-FFT SMAART based measurement rig and post the results.
Gabe