The tone controls are completely removed from the signal path in both the 6900 and 6500 when at the 12 o'clock position.
Quoting from the MA6500 manual,
"Bass and Treble Controls
Adjust the BASS and TREBLE controls to suit your listening preferences. The bass or treble intensity can be increased with clockwise rotation and decreased with counterclockwise rotation. All tone control circuit elements are removed from the signal path when the controls are in the center or flat position."
Quoting from the MA6500 manual,
"Bass and Treble Controls
Adjust the BASS and TREBLE controls to suit your listening preferences. The bass or treble intensity can be increased with clockwise rotation and decreased with counterclockwise rotation. All tone control circuit elements are removed from the signal path when the controls are in the center or flat position."