Ok, I see. The issue is mainly - you don't know excactly how much it takes to destroy the speakers. You need to work this out before making a plan how to protect and how much protection you're willing to put into this - and if it's worth the effort at all.
However - there is a couple of things you may do to prevent damage:
- Avoid clipping. That's probably no #1 thing known to destroy speakers. If you're to make a protection device, you must work out the maximum voltage out from the amplifier (need to see the audio IC datasheet for that) and still you should have some safety margin. Then you need a comparator and set up a fixed voltage, then a latching circuit that triggers a relay and the relay contacts connected to speaker ouputs.
- However - it doesn't protect if the input signal is already clipped. For this, you may opt for an opamp derivator circuit. From that you just follows the same steps as above.
- If electric effect over time also should take into account, you may opt for a current transformer between main transformer and rectifier. Then feed into a opamp based integrator circuit (must rectify this current first). But - first you must work out the power losses in the amplifier and also a sort of power over time curve that the speakers are guaranteed to withstand.
So - it is doable. The question are : Does the speaker cost so much that it is worth spend a lot of time and effort (and cost) to build this extra protection devices? And - is there enough space inside the amplifier cabinet to put another PCB, relays and current transformers?