First, congratulations on solving your problem. I especially appreciate your debug methodology - nice work.

It would have been an interesting experiment to try a ferrite core at the input to the RF-250, while leaving the source of the noise (the cable bundle) intact.... just to see the impact of the ferrite core.

On a separate but related note.... Since your installer may have been relatively inexperienced, you should probably recheck your installation for other inappropriate cable bundling. Such as signal/speaker/power cables bundled together. It is best to keep these bundled separately from each other. I bundle power cables together, line/signal cables together, and I leave speaker cables and remote controller cables separated from all others. For remote control equipment, I run separate routes that are not parallel to any other cables (well, best effort anyway).