I agree with Brian (as usual
).
I would try the "18.5-inch Reflector" mod first, as it only takes a few minutes, does not involve opening the CM15A (or even unplugging it - if you are careful), and does not void your warranty.
Just attach an 18.5-inch (approximately) length of stiff wire (a straight piece of coat hanger is fine) alongside the CM15A's antenna, lining the bottom of the wire up with the "elbow" at the bottom of the CM15A's antenna.
Attach the two together with a few wraps of tape, some plastic zip-ties, a piece of heat-shring tubing, chewing gum, etc. You are NOT making an electrical connection between the two. The longer wire is a reflector that helps catch the RF signals, and "bounce" them to the CM15A's antenna.
Many people (myself included) have had good success with this simple mod (others reported some success, but the range still wasn't increased enough to reach their farthest units).
It only takes a few minutes to try it, and you don't have much to lose (just don't poke your eye out with that long wire).