As I build a few amplifiers after nearly blowing up a speaker after one had a fault I came up with speaker protect circuit.
It just drops out a relay if it detects dc in the signal to the speaker.
I used a PIC12F509. I used 2 pins to detect the phase of the audio signal.
I also added a power up delay for the relay.
I then progressed the design to a solid state relay with dc detect, over current detect and power up/down detect.
I designed the pcb myself using PCBCAD360.