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