There is a common output stage in the XTB-IIR that drives both phases, and it is very unlikely that the L-C coupling circuit to the second phase is not working correctly. It is possible that there might be a bad connection to your second phase. You could VERY CAREFULLY verify there is 240V across the Phase I and Phase II terminals with well insulated meter probes.
While it is unlikely that any signal sucker could reduce the signal that low near the XTB-IIR, it is possible that a severe signal sucker might do that out on one of the circuits. But it would take more than the common signal suckers, such as surge protected power strips or computer power supplies.
You could check the signal near the panel on the second phase. If it reads low there, I would really suspect a bad connection to the second phase.
One of my other customers reported a very low signal level, and it turned out that his electrical system included some sort of electrical device that added a huge capacitance directly across the powerline to provide a clean waveform. As I recall, that device was supposed to improve the energy efficiency of his home. His only option to run an X10 system was to remove that device.
Jeff