I would like to point out from the beginning that I have not every tried any other portable amp. Also, I am using them with a very entry level pair of the headphones (monoprice over the ears DJ). A coworker bought the Fiio E02i, so I naturally was curious as to what it would do. While I was expecting more volume perhaps at the SQ, what I found was a substantial improvement overall. It makes my headphones sound twice as good. I naturally purchased my own. I have no doubt that the limitations of this amp would likely be obvious with quality...