Here's a question... WRT speaker isolation stopping DC blowing the speaker coil, this will not save the amp module although this will be broken anyway. It is also further complicated by the fact that (if I understand correctly) the bias on the electronic 'relay' needs to be held at a specific voltage below the signal level to ensure 'fully on' status and so has to be a fully floating supply. Therefore, I'd wondered about using something similar configured as an in-line fuse, i.e. built so that there are two (one per rail, between the cap bank and the amp board) driven by the same circuit and if DC appears on the output, then it will cut both pos. and neg. supplies to the amp board. This would keep it out of the signal path, introduce only a slight increase in the supply impedance (rather than between amp and speaker) and wouldn't need a fully floating supply. It would also stop the self-destruction of the amp board after failure. Thoughts?