If you can afford it, I would highly recommend an aftermarket head unit. You won't lose features (e.g., steering wheel controls, backup cam) as long as you buy the appropriate modules. Below is an image of what the signal looks like from 0-20 kHz on the stock headunit. Ideally, you would want to start with a flat signal. Notice the dropoff at the low frequencies (left side), that is the stock bass protection and you won't be able to recover that. One thing you can do if you really want to keep the stock headunit is get a DSP that has an onboard EQ so that you can flatten that signal. But honestly, with how cheap head units are these days it's better to get an aftermarket one and get a ton more features.