I have, however, just added a Hsu TN1225HO sub and it has no problem keeping up. I've been trying different subs but many have a hard time keeping up with the volume level that the Klipsch's can output. They play loud down to around 36Hz (measured in my media room). The speakers really don't NEED a subwoofer.
A kick-drum, for example, includes a higher frequency attack that the Paradigm's could just not duplicate. I believe that the quickness has to do with the horn going down to around 1600Hz. I compared the Klipsch to Paradigm line and found that the 4.5 was "quicker", more dynamic, and played louder. I've owned the Klipsch KG 4.5s for about a year now teamed up with a matching Klipsch center channel speaker. I give this modified speaker 5 stars (taking its price into account). Having paid $1250 in total, this speaker plays better than any existing speaker within this price range. The CFAC inductors, with their low loss made the bass notes more clear (before this modification, I couldn't distinguish a bass' string from a bassdrum, but now I can). Thanks to the MIT MultiCap capacitors the higher frequencies were more clear and "alive". The reduced volume makes the speaker's lower frequency limit a little higher, but the weight from the sand and the better filter parts makes the total result a tighter, deeper bass sound!! I spent 5 hours of work and $250 on high quality filter parts and cables. The solution to get the speaker to perform well was sand in the bottom of the box (3 liters/spkr with a thin wooden protection to hold it in place) and better filter parts. Nothing to add extra stiffness to the box!! The filter was dimensioned with very tiny cables and poor quality parts!! Since I hadn't got the money for the Thiel's, I opened the speaker (warranty void, of course) and found out that there's just open air inside the box. However, when I tested them against some (higher priced) Thiel speaker, I knew that they were passé. It sounded very good to my ears at the time. I bought a pair of KG 5.2's about five years ago.