Oppo HA-2 - Manual (Page 13)

For certain IEMs, if at the low gain setting the volume is still
too loud, you can adjust the volume control on your
playback device. For Apple products connected to the
USB A port or playback software that supports USB
volume, the HA-2’s internal 32-bit digital volume
control will be usedso there will be no loss of resolution
or truncation of audio samples.
CAUTION:
Changing the gain level on the fly may result in
a sudden increase of the output power, and may damage
your headphones and hearing.
Bass Boost
If you like to listen to music with a strong bass effect, you may
turn on the Bass Boost function of the unit. To do so, flip the
Bass Boost switch so a green bar is shown.