Strange listening situation

I have a McIntosh MC202 amp and a McIntosh C220 tube preamp with a built in headphone amp. Here's the strange thing that is happening. When playing this one song using both the amp and the preamp I cannot hear the vocals on the song. If I turn off the amp and use the built in headphone amp then the vocals come through loud ands clear. Could this be an issue with the tubes in the C220? It only happens on this one song. Any ideas?