Sub-woofer fast enough for Horns?

Sorry guys, my horn is Cessaro Chopin. Their smallest model. I think it goes down to around 30Hz on paper. I am very happy with most music but not enough for electronic and heavy rock. I had Thiels 3.6 before and I think the Thiels has stronger bass.

Technically subwoofers don't need to be fast as the frequencies they do are relatively slow. As far as the mass argument, it's more the ability to stop than go. Rythmik uses a servo system that allows the driver to be a bit lighter but, usually, heavier means more extension and larger means more for less excursion, where too much leads to distortion.

A sealed sub will have a fraction the group delay of a ported or PR sub but sacrifice deep bass with an earlier, shallower rolloff. Not usually a big concern for music and subs 12" and over, considering room gain. For the HT bass fanatics, it's another story.

Have you considered an active crossover, one that has a high pass for the main speakers? Maybe something like a Paradigm X30, Reckhorn S-1 or miniDSP. Maybe even a Harrison labs FMOD inline passive. How about a DSPeaker Antimode?

The advantage of field coil or em type driver motors is these can be made more powerful than natural magnets and make for a faster drive when needed, almost always for large bass drivers.

An alternate approach are very light but rigid drivers like the paper drivers in triangle titus monitors, which are often praised for their "speed", but they are small and have limited extension. Scaling to larger drivers for more bass this way is a challenge.

Rel has not the fastest response in the market. In the past I also sold the more expensive ones. To be honnest they were in some recordings too slow. With Audyssey Pro with fast sub's you now can get a fast and full stealth integration. A few years ago I thought this never would be possible!