DC Offset on the output is the most likely culprit
2 responses Add your response
Agree on the possibility of DC at the amp outputs. If you have a digital multimeter, you can check the amp for DC. Make sure you have nothing connected to the amp's inputs and outputs.
Set the DMM to the VDC scale initially, and connect the probes to the amp's + and - output terminals. Turn the amp on and take a reading. If you get no reading, then switch to the mVDC (millivolts DC) scale and take another reading.
Ideally, DC offset should be less than ± 15mV. DC at the output of several volts would cause the damage you've experienced.
One solution to safeguarding your speakers with an amp that has no DC protection is to retrofit DC protection circuitry. I've installed such protection in my Phase Linear 400 Series 2 amp.