Yes I'm definitely going to try to get it to happen again, but not with my earphones in this time! It was on auxes 1&2 and 2&3, and it happened as I plugged a cable in to the aux output.
The stereo interference is hard to describe, in that it just doesn't sound as clear as the mono signal. It's like old radio interference, a wet sort of background noise. I wonder if it's just the stereo quality of the transmitters, it's not like they're high end or anything. I have contacted Ld systems to ask their opinions.