I like the Cornwall's sound-dynamic, musically engaging, full (not thin in the upper-bass) and good at lower volume levels. Like many horn systems, it has a certain tonal qualities and some roughness in its frequency balance that one either loves or hates (I am more in the "love" camp.
The Double Impacts I heard had a much more modern sound--even frequency response. But, unlike many modern speakers, it did not seem dynamically flat and unengaging and it did not go completely dead at lower volume levels In that sense it was much more like better high efficiency/horn systems. It may not have as much of those charming qualities as the best high efficiency/horn systems, but, it is very good nonetheless and it has fewer of the tonal anomalies that can plague high efficiency systems. It is a very good all around system and it is quite a bargain at its price point.
In short, the Cornwall is more toward the you either love or hate kind of sound, the Double Impact is more balanced (I don't see too many people hating its sound).