AKG 701's maximum power input 200mw at 1hz.
Asus Xonar STX power output 3.52V @ +12dB for headphones with impedances of 64–300 ohms
• Power (P)
• Resistance (R)
• Voltage (E)
Calculate for Power (P)
P = E^2 / R
3.52V^2 / 62ohms = 0.19985 watts (199.85mw)
There you go, almost exactly 200mw (at a load of 62ohms), the maximum input power for AKG701 recommended by AKG.