I would say the issue is the fact that you are using the aux jack.
Does your player have an line output or are you going from the headphone jack? If headphone the sound is being converted in your player then amped. That amped signal is now going to your car stereo then being amped again. What volume level are you at? I find setting it to about 75-80% yields a nice strong signal that can be raised on your car stereo without adding distortion.
I've never been a big fan of aux inputs due to having to adjust volume in 2 places to ensure good sound.
It has line out that I plug into my CD player's aux and it also has a separate headphone jack. And it also has a coaxial out but I have no idea what that's for. I also would rather not have to use aux but your choices are limited when your not using a drm encumbered proprietary music player. My CD player does have a USB port that will play an iPod but I think it was specifically designed to play an iPod. I don't think it will play my iBasso.