Well, the scientific method usually comes into play about here. If you have other equipment/cables available, start swapping one out at a time, and see whether the issue gets resolved. Can you plug your AD 20 into something else? If so, and nothing happens, there is your answer. If there is signal, then move on to the cables. Do you have another SPDIF source to try directly into the MT? Try that as well. Just some thoughts.