DF will not control the driver excursion. The driver will move somewhat with inertia when the signal stops regardless of DF. What happens is that the moving driver acts like a microphone as the cone moves in the voice coil creating an emf. If the amplifier DF is very high, it's low output impedance presents a short to the speaker. This prevents a back emf to the speaker which could cause the driver to oscillate as it repeats the process.
What stops the cone (or controls it) is the air mass loading on the driver from the speaker enclosure (Q). Also, the crossover and the voice coil add to the overall DF in addition to the speaker cables so the amplifier DF will change once hooked up to a speaker. And with a planar speaker, the air loading is quite high which helps things out with low DF amps.
Then there are the unavoidable compromises in amplifier design: for an amplifer to have a high DF it has to have a very low output impedance which then will require more global negative feedback. This creates another problem to deal with in terms of distortion. Bottom line is that numbers don't tell you much.
What stops the cone (or controls it) is the air mass loading on the driver from the speaker enclosure (Q). Also, the crossover and the voice coil add to the overall DF in addition to the speaker cables so the amplifier DF will change once hooked up to a speaker. And with a planar speaker, the air loading is quite high which helps things out with low DF amps.
Then there are the unavoidable compromises in amplifier design: for an amplifer to have a high DF it has to have a very low output impedance which then will require more global negative feedback. This creates another problem to deal with in terms of distortion. Bottom line is that numbers don't tell you much.