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"Audio Power Amplifier Design Handbook" 2nd Edition, by Dougles Self has a chapter (12) devoted to this subject. This is the best reference that I have found.
. "Designing Audio Power Amplifiers" chapter 15 by by Bob Cordell
. "High Performance Audio Power Amplifiers" by Ben Duncan
Most loudspeaker protection circuits usually involve some form of DC detection, and relay isolation. It's difficult to define the point between reliable protection, and false triggering.
One alternative idea was adopted by QSC some years ago, which removed the relays -and the false triggering problems - by AC coupling the output. Effectively using the DC supply decoupling as output DC isolation. Very ingenious! Typical circuit schematic here : **broken link removed** if you're interested.