This thread has an astounding jump factor. Four years between notes is a long time.
This is a very interesting topic but, as many have said, it screams for a definition of jump factor. This is a very personal thing but to me jump factor is that which startles the human psyche, which to me is a function of the velocity of SPL rise at the listening position. To me this is also somewhat frequency dependent, the "startle" being inversely proportional to frequency for a given SPL rise velocity, but I am guessing this is a also personal thing.
If you agree with this view it seams to me that amp speed (rise time, capacitance, instantaneous current etc.) are as important as a speaker's ability to launch the waveform with scale and velocity. With this definition it is hard for me to agree with comments here that jump factor is about realism or even fidelity and not about speed and ultimate SPL. The most startling events in my room come in passages where SPL goes from 0 db to 100 db instantaneously.