I ask as a friend bought a stereo power amp then added a 2nd one recently - to have them both fail within hours of each other. He reports no obvious mains issues. Am not sure what the nominal impedance of his speakers are though. These amps are designed to optionally be used this way so have a switch on them. He tends to listen at moderate volumes. The dealer has checked for blown internal fuses - all were fine. They have been sent off for repair. These amps were being driven straight from a NAD M51 DAC.