Hi Tyimo,
I don't believe a servo can replace a proper protection circuit. If for example a faulty output device shortens the output to either main rail, the servo will not do much about that.
If you have a DC servo, it might be easy to do the following:
- Add a relay that, if closed, grounds the output terminal (so it is not in the signal path)
- Add a comparator that closes the relay in case the servo's integrator approaches the rail voltages, i.e. when the servo can no longer keep the offset at 0V.
That solution is easy to implement and will protect your speakers. It depends on the design of your amp (is the output current limited, does it have fuses in the main rails, etc.) whether it will work without causing too much damage to the amp.
Coming back to your question about sound quality: A servo which is not designed properly can degenerate the sound, and so can a relay in series with the output. But fuses in the main rails can well, so there is no panacea I guess.
Hope these ideas are helpful for you.
Sebastian