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The KYBD AMOUNT dial is part of the filter section of the pro-1 and determines the amount of control the keyboard has over the filter. 

The higher the dial is set, the less filtered the higher register notes are meaning the timbre is a lot brighter. When set lower, more of the higher frequencies are supressed by the filter creating a duller sound. When set between 7 and 8, the filter cutoff frequency is maintained at a constant point relative to the notes played on the keyboard, creating consistent tone.