iTunes - what does it do to our music?

[paul_g]

I notice that when I mix in Pro Tools and play back in a car stereo, home stereo, cd player, iPod (ear bud) etc.. or play it back using quick time through my 003 and monitors, my mix is very consistent.

The minute I bring any track into iTunes, whether it be my mix, or another commercial grade CD, it just sounds really different. The mix itself becomes thin (opposite of wide - not referring to bass) and very compressed.

I turn off all the enhancment junk in iTunes... etc. What exactly is going on under the hood of iTunes and why does music sound much different when it is played through iTunes, vs - car, home stereo, quick-time etc? What are they (Apple) doing to it?