The 0db setting is normally set to the point where the camera has the greatest latitude. Reducing the gain can reduce the appearance of grain but it will also reduce the dynamic range or latitude of the camera. So use negative gain with care!

If Sony's Z cameras are so bad, why are they so popular, why did the BBC, who can choose any camera they want and extensively test any cameras before approving them standardize on the Z1? I had a Canon H1 for a while, great camera, liked it a lot but I didn't find that Canon's implementation of HDV was and different to Sony's.