For general music, the Kanta 2 might be okay without a subwoofer. I haven't had direct experience with these, but I seem to remember they are kind of light on bass (I could be wrong).
The REL subwoofers are extraordinarily clean and smooth. If you want really good resolution, they are excellent. If your primary purpose is music, they might be a good choice. But they are not meaty sounding subs and if your primary purpose is home theater, I would look at a really good vented sub, like something from Rythmik. There's a definite difference in sound between a sealed sub and a vented sub
The REL subwoofers are extraordinarily clean and smooth. If you want really good resolution, they are excellent. If your primary purpose is music, they might be a good choice. But they are not meaty sounding subs and if your primary purpose is home theater, I would look at a really good vented sub, like something from Rythmik. There's a definite difference in sound between a sealed sub and a vented sub