I have done this and much like everything else, it will sound fine at 1st but eventually "blends into the car" if that makes any sense. Initially I thought this would be my best option because at baseline with everything both in the MP3 Player (Sony NW-A45) and the car, I would then engage the Equalizer setting on the player I use for headphones and then use the Bass increase in the car to fill that out a bit for a balanced sound. Initially it will sound fine, even with Surround feature on. But after a few drives or stops he sonics flucuate. The best way I can explain it is in my 2012 Sonata in which I actually kept all factory audio installed, I knew exactly what my Volume settings were for Driveway, In Town, or Highway; and it was like 2 point spread between each (8, 10, 12) for example. Yes I am aware digital files have their own sound field and fluctuations depending on age, production, etc so that isn't what I am up against. This car I cannot even land on a comfortable Volume environment because it goes up and down from drive to drive. The car has some sort of "intuitive" feature that modifies the sound field based on preset conditions and it doesn't matter what audio source you are using. Hard to explain but true.