Sweet. i'll bump until you get it all situated. So the predominant factor in transient response is speaker inductance-the time a vc stores the energy before releasing it and converting the ac current into force, thus producing/initiating the motion of the driver.
Here's the issue (to which you may or may not have been alluding)- Conventionally, transient response refers to the ability of a speaker to respond to any sudden change in that signal without blurring the sound. That is, how much time elapses before the driver responds to the input signal/applied voltage.
When reviewing the more classic definition, transient response affects reproduction of the attack AND decay characteristics of a sound. So here (where subs are concerned), the response consists of a brief, quick i½attack" followed by a relatively retarded "decay". The latter of which may plausibly include overshoot and ringing.
So, while mmass has no effect on the conventional definition of transient response (transient attack), it would make sense that it possibly impacts the decay or ability of the sub to recover from the attack.
i know you're in the business of testing and the ilk, and was wondering if you might be privy to any equipment required for this one. (Praxis, Test mic?)