Sometimes amps do blow because of the feedback
used that was intended to be beneficial. This can
happen when the output stage cannot handle the
creation of continuous full-scale AC output, and
the amplifier becomes unstable once the feedback
is applied, then oscillates at or near full-scale.
Nearly all the amplifiers that make it out of the
development lab are designed to be stable with
the feedback that is used. And many amplifiers
are designed to drive their load at full scale on
a continuous basis without self-destructing.
Internal feedback is ordinarily set up so that it cancels some of the
normal input signal, effectively reducing what is left to pass through
the amplifier. This is called negative feedback because of that
cancellation. Since the feedback comes out of the amplifier, if the
amplifier is not perfectly linear but amplifies parts of the input
signal more than other parts, the parts that were amplifier more
produce mode cancellation, making the net result more linear.
Positive feedback is that which adds to the normal input signal,
making it bigger, and also increases the distortion. If positive
feedback is strong enough, it will make the amplifier an oscillator,
and it will make its input with no help form any other signal. This
kind of feedback can seriously overdrive the amplifier and if that
produces excess heat, can overheat the amplifier.