op-amp

I just got this Amp_Dac for my DT990 250ohm headphones: http://www.monoprice.com/Product?c_id=120&cp_id=12008&cs_id=1200801&p_id=11567&seq=1&format=2 It is a remodeled Fiio E09K + it has a DAC in it, so it is quality made and is suppossedly 15-600ohm I also see "Output Power" on the specs of the...