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...