This switch tailors the high-frequency response by ±2 dB at 3 kHz and above on all three MR models.Leave this switch in the “0 (NORMAL)” position unless:• You want to subtly brighten or darken the sound of the speakers.• Perhaps you have hearing loss caused by too many nights in front of a Marshall full stack.• You just like to mix on the bright side or dull side.If the mixes consistently sound dull or dark when you listen elsewhere, this usually indicates that the monitors are too bright, relative to your normal hearing.