I found the Benchmark Application Note regarding Damping Factor very interesting and written in a way that's easily understandable.
Specifically, how characterizing Damping Factor, or amp output impedance, with a pure resistive load does not tell the complete story of driving a moving coil speaker with resistive, inductive, and capacitive characteristics that change with frequency.
https://benchmarkmedia.com/blogs/application_notes/audio-myth-damping-factor-isnt-much-of-a-factor
Specifically, how characterizing Damping Factor, or amp output impedance, with a pure resistive load does not tell the complete story of driving a moving coil speaker with resistive, inductive, and capacitive characteristics that change with frequency.
https://benchmarkmedia.com/blogs/application_notes/audio-myth-damping-factor-isnt-much-of-a-factor