I use the factory head unit and get excellent results. I upgraded the speakers, added a sub/amp combo, and finally amped the speakers with a 4 channel amp - 40 watts per.
Got good, but not great sound. 100% better then stock and something most people would be happy with.
To get the extra clarity, I added a Navone N774V adaptor. This is basically a LOC with a gain control to get higher input voltage from your stock head unit to the amp. Goal was to amplify the signal and not the noise in the signal.
This gave me the sound I was looking for & more. I found that the sound was as good as anything I'd heard at local audio dealers. It allowed me to bump up the signal before the amp gains, so I could run the amp's gains at a lower setting. This eliminated any turn on/off noise or amplified internal noises from the deck - like when seeking a track on the cd.
My goal was also to retain the factory look of the dash. The red/amber interior lighting on the ES is an outstanding feature of the car. I wasn't willing to give that up, unless I had to.
Fortunately, I didn't have to.