If one has mid level mains, SVS or Rythmic are fine but they won't be fast enough to do the high end Focals justice. Speed is not about how fast they make sound but actually how fast they stop making a sound and get out of the way again. Power and grip is what's needed to make this happen and unfortunately you get what you pay for.
This very thing you speak of is why Rythmik subs incorporate servo technology.