CDs playing much better on car stereo than mp3 player?

[imported_Christopher Greene]

I recently had new after market alpine speakers installed and an alpine amp as well. I noticed when playing the mp3s they sounded more hollow and I needed to adjust the volume to 23 for decent listening. When popping in the same cd it sounded much more full, higher quality, and only needed to turn it to 11 for great listening. Is it typical for CDs to sound better or is there something I need to fix? Also, the mp3s are high quality and kbps is also high on them. I tested out multiple mp3s and cds at the same time. Any help is greatly appreciated.
Another thing is I turn my phone mp3 player up all the way.