Irish 14 - I have the C2 helmet and Shuberth bluetooth system connected to my Zumo 550 and cell phone. Very happy with it. With the previous Cardo Q2, I had it at 90% volume, with the C2 setup, I have it at 30%volume for the same result. Cardo ( who made both systems) appear to have made a couple of jumps in the technology. Currently on mono, but will try to set up with my Zumo 660 at some time. Actually, I don't really mind mono, so long as I can hear the music.