I recomended protect with relay, but triac DC protect for speakers is better than nothing. DC offset over than +/-35v will demage speakers no metter if output devices is failure or not. This triac protect connect after you sure there is not DC on amplifier output, and then you can connect speakers. At the end Quad405 have this kind of protect.
Regards

http://www.diyaudio.com/forums/solid-state/164208-500w-pa-amplifier-limiter-8.html

This choke coil is not critical 12-14 turns, 1-1,5mm wire on 15mm dia coil.