The damping factor of amplifiers is a measure that has fallen out of favour in recent years. Many brands don’t specify it any longer. I had long considered it an important specification, since a low damping factor can lead to frequency response anomalies and poor control over the bass driver in a speaker. But, like an idiot, I failed to take into account that the effective damping factor of an amplifier and speaker system is based on the internal impedance of the amplifier (I knew this), the resistance of the speaker cables (I knew this as well) plus the resistance of the speaker voice coils. It was this last that I had overlooked.
No excuse: the whole thing is explained in an ancient (1967) paper written by George L. Augspurger from James B. Lansing Sound Inc. Read the whole thing.