In the manual for operating the remote, page 9 of the PDF file:


T.
STEREO/MENU button
(see page 39)
This button deactivates any active surround modes and
switches the Model 975 back to stereo operation. If a
subwoofer is connected and configured through the
setup menu, it will still be active in stereo mode. This
button also operates the Menu function on devices that
the remote has been programmed to control. Note that
it does not operate the Model 975’s on-screen menu.