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?
jimmymac
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Believe me, I know this sounds strange. Mafuta, thanks for the response. I will check those things.
Mafuta, you had the solution. I had the amp switched in bridged mono operation. I set the switch to stereo and the vocals now come through. I recently changed to XLR ic's from my preamp to amp and must have inadvertently hit the switch to bridged mono. Thanks for your help!