You might find your sound will improve even more when using SD cards loaded with FLAC files. Your car has two SD card slots.
Today I was playing with the sound listening to Sirius Radio and noticed that it has specific sound settings for Sirius as well as FM. Today I had it treble at -3 and bass at 0 on Sirius XM. On FM, I turned the bass down -2 and treble -3, but it can change slightly depending on the source of music. As I said, the base radio is not terrible, I am happy with mine but last week I was driving around with my 16 year old daughter and she plugged her iphone into the cayman. She started to play music from her phone which I hadn't done and it sounded really great. Much better than the FM or XM. I was really blown away because MP3 is a compressed lossy format. I had never played iPhone music before and it made the radio sound even better.