This is from ADI-2 PRO manual: Balanced Phones Mode
Output level is doubled. With the ADI-2 Pro +22 dBu would rise to +28 dBu (a gain of +6 dB). Now only few might have a headphone that requires that level, a scary 19.5 Volts in output voltage. But when the phone is driven at the same volume and effective level as before, the driver stages now operate at a 6 dB lower level. This can have a positive effect on THD and linearity. Output power is quadrupled. A driver stage that was designed for 1 Watt will deliver 4 Watt when a second such stage is used with inverted input signal. That is a significant rise, which would allow to either achieve output powers that even bassheads would fear, or allows for the use of less powerful or simpler output stages than usual. Complete ground-free operation will prevent EMI and floating voltage level problems in certain (seldom) cases.