Sub-woofer fast enough for Horns?

What kind of horns are you trying to blend with? What is your crossover frequency? I use the tact w210 subs (3 pairs) to integrate with my avantgarde uno's with superb results. The subs are now discontinued but lyngdorf makes the bw-1 which is similar but not quite as good.

You should go for Cyclone subwoofer or Open buffle design sub may work fine as well.

Regards

Yea - give the Martin Logan Depth I a try - designed to match the speed of Electrostatics.

I have one myself for my very fast mains (they are not horns but have tricks done like lining with 1/4 copper to decrease resonance and increase speed) and their speed is fine. The only issue is, as with any subwoofer, integrating it into the mains seamlessly. I personally only ever use it for HT because the music I listen to doesn't have much low bass. But when I do engage it for music, while I generally prefer it engaged, you can detect some anomalies caused by the fact its not seamlessly integrated - its speed however is fine.

Thank

What makes you think horns are 'faster' than anything else? Empirically horns would measure slower than any other type of speaker due to all the stored energy from reflections inside the throat.

My suspicion is that the Rel sub was just fine, it's the integration that needed work. Suboptimal placement will give the impression that the subs are too slow, the sound will appear to lag behind the impulse from your mains, when reality is small placement changes is all that's needed.