The so-called speed is what's perceived when there's a lack of distortion. I've heard the EP400 in my home and there's no audible distortion. Unfortunately I can't say the same about my EP600 v2. I am sure however the new line of Axiom sealed subs is distortion-free.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated