[Kyle Splittgerber]

What's the Monitor Path assignment in the bottom right of the Output tab? Try changing that to MON L-R if it's not assigned to that output. If it is assigned to MON L-R, try assigning it to 'no output' and/or a different output path and closing IO Setup. If that works, go back to IO Setup and assign the Monitor Path to MON L-R if necessary.

If that doesn't work, go to Hardware Setup and switch the Headphone Sources to each individual headphone (HP 1 : HP 1, HP 2 : HP 2, etc) and disable Main monitor set, then default IO Setup. If that works then go back to Hardware Setup and re-enable Main and Headphones Sources as desired.