For normal use, leave it as it is.
If you like to try, you can easily go back to normal afterwards.
Read here, manual page 28, 29:
14.1.5 Expert Settings
This menu only exists for ADI-2 DACs with an ESS DAC chip. It provides access to specific configuration parameters of the chip, such as THD compensation and fine adjustment of the output level. It is mainly intended for engineers and technicians. All settings are automatically stored in the device and automatically loaded when the menu is activated.
Enable Settings
OFF, ON. Default: OFF. When activated, the other setting options are displayed. OFF activates the factory settings.
THD Comp K2L (K2R, K3L, K3R)
The ESS allows the adjustment of a correction factor to reduce the second and third harmonic, i.e. a reduction of the harmonic distortion. The THD Compensation is adjustable in wide ranges, where 0 corresponds to the RME factory setting.
Very high values (10.0k) cause around -60 dB distortion (0.1%). It is therefore also possible to use K2 and K3 separately and intentionally as an effect. This can be quite instructive - when one notices that no difference between 0.1% and 0.0001% distortion is audible.