Question Why can I not hear the double on the Harmony G-XT when the double switch has been engaged

The harmony volume is also the double volume. To confirm the double is working, try this:
  1. Turn off the harmony 1 & 2 buttons
  2. Turn the harmony volume up all the way, clockwise
  3. Engage the double button
  4. Try the vocal.
You should now be able to hear a distinct double.