If there´s one disadvantage of mp3 it´s that its short block window size is too big, leading to smeared transients on occasion. This will happen with low as well as high bitrates. http://www.mp3-tech.org/content/?mp3%20limitations AAC or WMA completely avoid this in comparison. But I´ve yet to manage to encode to working AAC (the X3 somehow ignores my encodes) so I´ve opted for WMA Professional for the time being. Secondly, the FiiO doesn´t decode mp3 or aac with 32 bit floating point, despite the fact that this would be easily possible.