The specifications are quoted from firestone's website. Sadly even the manufacturer itself deosn't have any real information (gain, current output for various impedances, etc) Nevertheless, if it can output 2V RMS, it will be able to drive even an HD600 into "too loud" territory. (The HD600 has a sensitivity of 102 dB/V, according to headroom.)
As far as "reference level", that phrase is basically meaningless, but it should be able to at least "sound good" (that is, operate not too far out of spec, as far as distortion goes), as higher impedance headphones need little current, so the amp most likely wont be overtaxing itself. What's more, you'll have to have a higher volume (and more voltage) so the dynamic range will be much closer to the 110 db at the 2VRMS level.
That said, I probably wouldn't use a pair of HD600's portably.
if the "real" specifications were published anywhere, I'd consider buying this, but as it is, i'd rather just keep using my ancient cowon D2.