Mmmmm ..... this will not be a welcome post ....
It appears to be forgotten how damping actually works.The formula regarding (load impedance)/(amp output impedance + cable impedance + whatever else) neglects one important factor. It is said that the electromagnetic 'braking' of the movement of a cone is controlled by the resistance/impedance in parallel with or shunting the voice coil at any stage/frequency. To put it in popular parlance: How big/(low-resistance) the short across the driver is.
This seems to be the general concensus, but it is flawed - and which eventually made Fritz Langford-Smith, who seems/claims to have 'coined' the term, regrets his definiton somewhat later as misleading.
The point (Ohm's Law): 'Braking' current in the voice coil is governed by the
total resistance in the amplifier-driver circuit.
Then what about the voice coil wire resistance?? Why has that suddenly disappeared from the ohmic equivalent circuit? The lowest
total series impedance that could ever exist in that circuit is the wire resistance of the driver voice coil alone, everything else being zero. (Here I am excluding electronically generated negative amplifier output impedance - that is a different story.)
To sum up: Very generally the voice coil (wire) resistance of a driver happens to lie at some 60% of a driver's impedance; say around 5 ohm for an 8 ohm impedance voice coil (no empirical relationship; it just happens to turn out like that in practice).
That makes the highest practical possible D.F of an amplifier-loudspeaker system around some 1,6*. (Now one can really go make this complex by referring to actually more accurately impedances, proceeding to phase angles, influence of cross-over networks etc.etc.)
That the influence of (so-called, as defined) DFs upward of say 5 - 7 rapidly becomes irrelevant has been demonstarted by numerous practical tests and illustrated by as many graphs. Relevant articles are also plentiful; I normally refer to the popular 'westhost' site written by Rod Elliott. (This apart from the original article written by Langford-Smith .... rather long ago in an Australian magazine.)
This is rather at a tangent to the thread subject; apology. But it should be considered. And to hasten to add, hence the *; I also do
not discount effects on loudspeaker performance where l.s. impedances near the primary system resonance soars .... well, almost all over the place. My point is simply that, even more than Douglas Self intimated, so-called high DFs have no audible effect on the performance, mainly because of the wrong DFdefinition, and is simply of promotional value.
Edit: Typo