System Configuration : I/O Setup : I/O Setup Options : Default Monitor Format

Default Monitor Format
(Pro Tools HD and Pro Tools with Complete Production Toolkit Only)
The Default Monitor Format pop-up menu lets you select the default monitor format (Stereo, 5.1, or 7.1) for new Output paths and for when you click the Default button.
This setting does not affect existing path definitions or metering—it only specifies channel mapping in new 5.1-format paths.
The Default Monitor Format selector is only available in the Output page of I/O Setup.
To choose a Default Monitor Format:
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Select the channel mapping from the Default Monitor Format pop-up menu.