Flatness is not even. Our perception of flat frequency response changes with volume, which is why there isn’t as much bass or treble at low listening levels.
The problem with this should be obvious. If our goal is a flat response curve as concerns a meter’s measurement, our system will sound flat only at specific volume levels.
The cause relates to how we process sound. We’re not microphones performing linear conversions. Instead, our ear/brain mechanism perceives sound—which means we interpret what we hear. And what we hear varies with volume level.
Years ago receivers and even preamplifiers had loudness controls that pumped up the bass and treble at lower volumes in order for us to perceive flat response. An interesting paradox; at low volumes we must tilt the sound so it measures flat in our brains.
The world is not flat as much as our measuring equipment would have us believe.