No I don't know, but stick to headphones less that 300Ohms and you'll be fine. If the headphone amp in the UQ2 is any indication for the Atom, it will work find with 300Ohm units but struggle a bit to sound open and unforced.
But don't forget that like speakers and amps, it is not purely about the numbers. You need to audition to find a pair of headphones that has synergy with the unit and more importantly, physical comfort.
