Amplifier does not sound right when balance is centered
Reverse the polarity of one of the speakers. It sounds as though you have one of them reversed and so they are out of phase. Vocals and bass are usually centered in the stereo panorama and when they are played out of phase on left and right channels they tend to cancel out in the centre of the stereo image.

The reason for this is that one speaker is pushing the air while the other is pulling. The net result is a cancellation or sum to zero at your ears.