The frequency response specs of most headphones can generally be ignored, since it is not known how (if at all) they were measured, and the +/- dB tolerance for the range is not specified either. So, the manufacturer can basically make up whatever numbers it wants for marketing purposes. The frequency response of the Xonar might only be specified for 20-20000 Hz, but that does not mean that it will not be able to reproduce 19 or 20001 Hz. In fact, it is very likely that in practice the headphone will be the limiting factor.
The main limitations of the headphone output of the Xonar U3 are its relatively high output impedance, and low maximum current output (it uses an IC designed for use as a line output buffer, rather than driving headphones). Still, most of the distortion and frequency response error will probably come from the Q40, except perhaps at high frequencies the "headphone amplifier" in the Xonar may distort more than some headphones.
Edited by stv014 - 5/9/13 at 4:04am