Sound Issues Outside of Cubase

I’ve recently set up an signal chain that goes something like this:

Bass>effects>amplifier>Presonus Audiobox>PC

When I import audio files, or play my bass through the Presonus headphone jack, the sound is crystal clear. However, when listening to audio files via iTunes or the Internet, I get a very unbalanced, slightly distorted sound. Drums and bass soud thin, and the treble sounds way boosted, almost as if one side of the headphone is outputing more volume than the other. Again, the same audio files sound perfect in Cubase.

Any ideas how to fix this?