Asus Rampage III Black Edition - Manual (Page 168)

4-20 Chapter 4: Software support
Audio output
Click the corresponding box to select from Speakers, Headphones, and SPDIF-
Out for the output device. Click each button directly to configure the related
settings.
Headphone Settings
Click Headphone and select Headphone Settings to do the gain
adjustment. Select the gain tuning according to your headphone impedance.
A wrong gain tuning setting may damage your headphone. Select a proper gain
according to your headphone specification.
Only audio output from the front panel via the AAFP cable supports the
headphone gain tuning function.