The X10 Boost input is isolated from the powerline by a low-pass filter so the XTB-IIR can distinguish commands coming in there from commands coming in over the powerline. A return-signal bandpass amplifier feeds powerline signals to the X10 Boost input so a controller can still monitor powerline traffic. Some powerline noise can look like X10 signals after passing through that bandpass amplifier. Since the CM11A and CM15A do not incorporate AGC to ignore powerline noise, they can believe that noise is X10 traffic on the powerline.
There are two good options. The easiest is to just plug the CM11A or CM15A into an adjacent wall outlet, and use the XTB-IIR strictly as a repeater. Or if you have the latest firmware, you can reduce the gain of the bandpass amplifier to make it less likely that the CM11A or CM15A will be bothered by the powerline noise. But that also reduces sensitivity to X10 commands coming in over the powerline, which could be an issue for controllers located a long distance from the XTB-IIR.
Your solution would also reduce the amplified powerline noise by adding the filter's parallel-resonant tank circuit across the X10 Boost input.
We also make our own X10 filters, the XTB-F10 and XTB-F15. While also using a T-network like the higher current X10 and SmartHome filters, the we use different values for better performance. The schematic for the SmartHome product also does not include the bleed resistor that is required for UL approval. The schematic for the XTB-F10/F15 is on our website. http://jvde.us/xtb/xtb-f10_schematic_v10.pdf