It is not a device designed for phones, as can other products in the range Fiio. Here, no battery and entry is limited to USB (no line input).
It will ideally be accompanied by an external headphone amplifier like FiiO E11 but it is very good one also.
Gain control (low / high) allows the use of helmets in very different sensibilities. Bass enhancement is very effective without unduly distorting the music, covering low but also pushing a little mediums. You do not fall into the "boom boom".
Diehard audiophiles avid Internet tests will be able to read the processed signal is not 24 bit (closer to 16 bits) and the use of bass enhancement and gain "high" accentuates this phenomenon. In real conditions (ie with both ears rather than oscilloscope), rendering is very clean, the sound image is balanced and well defined.
The Windows setup is quite simple (think activate the 96kHz output) software side, Foobar2000 is ideal with Asio drivers Fiio or WASAPI (no significant difference between the two in terms of sound but my configuration is a bit WASAPI less stable).
Note that the quality of the USB cable used and the fact plug it directly on the computer or USB hub has a rather significant impact, something I did not expect a priori me.
Good buy!