Why does my music sound So bad?

Using a pair of Sennheiser 595s on my macbook pro 13", my music sounds like utter ***. I'm not exaggerating at all. These are extremely expensive hiFi headphones and they sound like god singing in your ears when connected to my iPhone, iPod, or stereo system. However, when i try listening to music via itunes on my macbook pro, the sound is muffled, the music is distorted, static can be heard, and it just sounds HORRIBLE. I have tweaked with my EQ and tried some special presets but to no avail… WTF is going on?

Note: all of my music is 320 kbps, so the sound quality is not the issue.

Your music sounds bad for the obvious reason: that a pair of flying squirrel monkeys sabotaged the truck which transported the goods causing it to be bumpy and damage the goods