I'm looking to add sub-woofers to my system...

I have speakers that are single-driver, very efficient and have terrific transient response.  My concern is finding reasonably priced smaller subs that have the ability to take input from the amp output and that have fast enough transient response to keep up with my main speakers.  I'm not looking for subterranean bass response.  Just looking to augment from ~50hz down to ~the lower 30hz range.  I have a rather small listening space so the need for physically smaller subs is important.  I'd appreciate anyone with experience, to help me with matching fast subs with fast response speakers...   Thanks !