Wayne Parham Messages: 18748 Registered: January 2001
Illuminati (33rd Degree)
It's more complicated than that, but autotransformers do provide impedance matching, whereas resistors simply impede current flow. The impedance interacts with the output circuit (tube/transformer) in the amplifier, which ultimately is the thing that provides the most electrical damping.
I think the most important thing to consider is the fact that low-power tube amps cannot provide much damping, so a loudspeaker that has wild impedance swings will cause frequency anomalies because of the voltage division between load and source impedance.