I have this exact problem with my Fiio E7 amp. When I use it with my iphone through the iphone doc the sound is amazing and the bass very clean. When I plug it in through the usb port on my computer, the bass gets fuzzy and not as clean. The equalizer on itunes is off so that can't be the problem. In the third picture that hendersondayton showed earlier in this thread, I have the sample rate and bit depth set to the highest possible for my amp.
If anybody can help me I would really appreciate it!
The iPhone dock is a line out using the iPhone's dac. The computer is using the E7's dac.
I guess its the E7's DAC that's causing the fuzziness.