I'm only using a single RFX-250. The second one was only used for testing purposes and in the end picked up far too much interference itself - but didn't cause interference to be received by the first unit.

As FP Crazy mentions, when there is any interferences that's likely to affect the RFX-250, its second LED will light up - dimmly or brightly. I am currently getting solid RF performance, but the lack of interference has not improved the latency issues. And I have managed to decisively prove that the latency is on the remote end at least in some situations by watching both talk-balk LEDs on other devices and listening for audio feedback on my pre-amp, which I've mentioned is picking up any cable talk on the MSC-400.