You can assign the MIX IN TD-30 to the PHONES output only. Doing this will prevent the signal coming into the MIX IN from passing through to the Master or Direct outputs, or the USB output signal.
Here's how to assign it to the phones output only:
1. Press SETUP.
2. Press F1 (OUTPUT).
3. Press F3(OTHER).
4. Cursor to "MIX IN," and then turn the dial to so that the "indicator" is on the PHONES line only.
5. Press EXIT when you're finished. The new setting are saved automatically.