I have reported this exact same problem. High's get distorted. Multiple music sources did it and on multiple playback devices. I could hardly stand to listen to it.
Then, magically, the problem has gone away a few weeks ago. I also had bluetooth auto connect problems... but those went away after I got rid of several bluetooth devices that were 'in front' of the car stereo. Not sure if this problem was related. It doesnt make any sense.
Google bluetooth audio distortion and you will see a huge amount of threads on this... Android 4.2 had a known bug about it.
Sorry I cant be much help but it's good to know that my phone is not the only one.