Perfect!
I've been long searching for an amp which doesn't change the timbre but adds low frequency response to my Hugo 2. This amp exactly meets my need. I tried some expensive gears like SPL Phonitor 2 but none was satisfactory. Those expensive gears sounds not bad but all of them adds distortion to the high frequency, whereas the 789 performs just like a powered wire!
A small con here: While using RCA input (Hugo2 is not a balanced DAC) and balanced output with HD800S, comparing to the single-end output the sound becomes more powerful but a bit harsh, which causes no problem because the single-end output is perfect to my ears.
Another amazing thing about this amp: It delivers large amount of power but I can't detect any background noise with high sensitivity IEM even at highest gain.