The metallic Rx connectors seem to have been made for flat cables. There's a little 'bump' on the clamp that will put too much pressure on the round cables if tightened even to proper levels. I think that it's a combination of the clamp and the way that the cables are made with thinner sheathing and insulation. I'd usually wrap a piece of tape around the cable first if I have to use these clamps with the round cables.
