I understand that at the low end of the frequency range, the resonance you guys are talking about is inductive and is damped best by a lower source impedance. But at higher frequencies, esp if there are crossovers and such, there may be other resonances, and impedances that look more capacitive in the relevant region. Source impedance will affect these in different ways, and with higher-order systems the damping factor is perhaps too simple a concept. Nevertheless, my point remains that the source impedance will affect the spectrum of the sound, in possibly complicated ways, and the traditional use of low driving impedance only optimizes one aspect of that, which may not correlate with the benefit that some say they get with added resistance.
It's worth looking at more closely.
Dick