One thing to remember is your in a car so prob not the best place to judge audio quality its always going to sound a bit harsh.
My home 2ch system (Quad + B&W) sounds a lot more balanced as its in the right environment.
ML home systems cost upwards of 15K so I'm not sure how the car version compares, but you do get a separate amp in the boot in the GS so its not just some upgraded head unit or badge.
The standard audio system is from Pioneer I believe.