The “Flatness” control adjusts the overall frequency response of the selected room-correction filter.
Most room-correction filters aim to make the overall frequency response “flat”. Based on measurements of sound in the room, they correct for peaks and dips (usually resulting from resonance in the room).
When you first use a room-correction filter, you may feel that the sound has become less warm, or more sterile, even as it improves precision. The warmth you’re used to is partly due to the very peaks and dips that the filter tries to eliminate.
The flatness control, then, puts the peaks and dips back.
