Whoa! XTB-IIR stuffed INSIDE the breaker box? Is the CM15A stuffed inside there, too?
If that isn't a code violation, it really should be!
That type of setup really scares me. When I bought my XTB-IIR, Jeff's recommendation was to install a dedicated 240V outlet, and wire up a plug to the XTB-IIR. That way, it is easy to disconnect if needed.
I strongly recommend moving the XTB-IIR outside of the breaker box, and wiring it up properly to a dedicated double-pole 220V breaker.
Once that is done, I suggest starting with the simplest of the CM15A antenna mods - the "passive reflector." Cut a straightened coat-hanger, or other similar wire (I used #12 electrical wire for mine) to 18.5 inches. Attach it alongside the CM15A's antenna, lining the bottom end of the wire up with the "elbow" at the bottom of the antenna. DO NOT make an electrical connection between the two. The additional wire acts as a reflector, catching the X10 signals, and bouncing them to the CM15A's antenna.
If that doesn't help enough, you might want to try the "F connector" mod, along with one of the other antennas.