My experience was that adding the XTB-IIR and a few filters solved my PLC problems for all but 1 module. I still have some noise generated by my dryer control board and my furnace control board. But, with the XTB-IIR and those other issues sorted by filters, these last 2 biggies were not enough to warrant the trouble fixing them. If there were an easy way to filter them without voiding warranties, I'd probably filter them too, though.
In my case, I picked an outlet that was closest to the panel on each leg and turned on breakers one by one recording noise levels and signal strengths using my XTBM as meter and XTB-IIR as signal source. Once I knew what circuits were problems, I then found the sources of noise/signal suckers. I picked the worst ones and filtered them (computers, entertainment center, refrigerator, washing machine, cell phone chargers). I had a pretty good signal everywhere, but found another few filterlinc's on ebay for cheap, so I went ahead and filtered a couple other items that were marginal (e.g., TiVo).
This is a time consuming process, but it does make a large difference in reliability.