[lae777]

To clarify (from the PT9 manual);

When exchanging sessions among different systems running Pro Tools 9.0, it is generally recommended that the Sessions Overwrite Current I/O Setup When Opened option be disabled. This maintains your custom system settings and
Pro Tools automatically remaps session output busses to the system output paths when opening sessions created (or edited and saved) on other systems.
When exchanging sessions with systems running lower versions of Pro Tools, it is generally recommended that the Sessions Overwrite Current I/O Setup When Opened option be enabled. In this case, any custom settings saved with the session that do not match your system may need to be reconfigured in the I/O Setup.

Good luck.