Fiio M15 - Over-Ear Headphone Mode; Lowpass Filter Mode

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Over-ear headphone mode
If you use high-impedance earphones (over-the ear), you can choose the Over-ear headphone
mode in the drop-down list to acquire ultimate experience.
Lowpass filter mode
There are 6 kinds of lowpass filter modes in the M15 which are the internal configuration of AK4499.
Any mode would be available for your preference.