Strange listening situation

Are you shure you're not trippin?
Retest sober please!

Vocals are usually panned dead centre in a stereo mix.That would mean that both left and right channels would carry identical signal for that vocal. If the two channels of your system is out of phase to each other the vocal will disappear. The signal is obviously still OK till it leaves the pre amp. Check your speaker wiring. If that is OK and you are running balanced connections check the pre to power interconnect cables. Also most Mac power amps has a switch for bridged mono operation. Check that that switch is set to stereo

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!