Those are probably 'hinge' points where the roll-offs begin.
3 responses Add your response
The knob does select the turnover frequencies for the tone controls. With the knob 'out', the tone controls are bypassed regardless of their settings. With it 'in' the turnovers default to the broader band responses of the day, bass frequencies below about 400Hz are affected by the bass control, treble frequencies above about 3KHz are affected by the treble knob. The tone mode knob allows you the option of adjusting just the frequency extremes for bass and/or treble at the turnover points listed. Current amps with tone controls usually default to these more-extreme points so as to have no effect on the midrange, despite there being little musical information below 100Hz or above 10KHz. You should be able to hear the difference at louder volumes and full-range spkrs by putzing with the tone controls/turnover frequencies.