If "PROTECTION" comes on under the circumstances you mention then that can indicate the possibility of a partial short circuit on the loudspeaker wiring OR by having an additional or alternative loudspeaker of an incorrect impedance coupled on to the system thereby overloading it. If this does not apply, then an internal amplifier fault has developed which requires investigating. (Try setting a little less BASS response, as that can sometimes make the shut down circuit back off slightly)