OK, so I've compared the HP output of Xonar ST and Focusrite Scarlett 2i4 with replaygained music or music that is properly mastered (= no replaygain necessary). The Xonar on high gain (+12 dB) is a little louder, but the gain of the 2i4 should be sufficient (9-10 o'clock knob position). Because the treble of the R70x is quite pleasant, you might find yourself in a situation where you would want to push them higher. However if you don't use replaygain, you usually have about +6 dB of album gain on average, which should be more than enough. I've never used a "very high gain" setting (+18 dB) on Xonar ST. From what I remember when comparing the Q701 and the R70x, the gain setting should be somewhat similar.
I'm not entirely sure if the R70x are good for mastering/mixing, because of their natural sound. They're what the music sounds like if you're listening to loudspeakers and are quite forgiving to badly mastered music. If you want detail/error retrieval HP, there might be better options. For example the MSR7 might be a better monitoring HP and they have very accurate treble, without sounding fatiguing.
