Actually Audyssey Pro offers three user-selectable Reference curves that can be customized by the curve editor.
High Frequency Roll-off 1 curve introduces a slight roll-off at high frequencies that accounts for the balance between direct and reflected sound for small to medium size rooms (room volume less than 2500 cu. ft.)
High Frequency Roll-off 2 curve introduces a slightly greater roll-off at high frequencies that restores the balance between direct and reflected sound for medium to large size rooms (room volume between 2500 and 5000 cu. ft.)
SMPTE 202M curve is an international standard for the high frequency roll-off applied in a typical 500-seat movie theater. It is appropriate for professional mixing spaces and dubbing stages that must be calibrated for film sound postproduction. It can also be used in extremely large playback spaces (room volume greater than 5000 cu. ft.)
When you select and edit any of these selectable curves, all curves, including the flat curve, is also automatically adjusted.
Refer to Chris' remarks quoted below.
Here's one of Jeff's adjustments:
The red curves is the customized reference curve. The blue curve with the handle boxes is the curve used to edit the selected reference curve. The blue curve also is a good representation of the new adjusted "Flat" curve. For Onkyo devices the blue curve would automatically be applied to THX listening modes, with the exception of Neural THX 5.1/7.1.