Dredging up this old thread to reply as promised about the HRTF on headphone outputs from personal playback devices.
Both people who I am certain told me about this don't remember telling me about this at all and deny that they believe this to be true in all cases.
One of them said there can be an issue with devices like this putting out multiple volts at 300-500kHz on the headphone connectors, but the device he named where he found it is not one that we'd likely find at a live sound event IMO. So there you go.
My apologies for spreading what is apparently a falsehood. As I said earlier, I am quite certain that I heard it and asked multiple follow up questions of both of them, as not using the iPod headphone output through speakers was and did immediately change my SOP a lot.
I'm sorry for any problems that were caused. This is a very strange episode.