What I would do is check for DC offset at the speaker terminal outputs. Set a DMM to DC Volts and measure across the speaker cables at the amplifier binding posts with the amp on (for at least 5 minutes) and everything connected as normal, all on. Then turn off everything, disconnect the interconnect from the preamp. Turn on the amp, wait five minutes and measure again. If there is more DC with the preamp connected, that may be the issue. If the DC is the same in both cases then it's the amp.
If there is DC offset, it should not be more than a few millivolts with today's designs. If there is more, say 50mV or more, that is an indicator of a problem with the amp. Protection circuits should not activate if there is no problem with the amp so you may be looking at sending it to the manufacturer for diagnosis.