For me it seems to be more to do with which IEM I have. The case might not be shielded perfectly but I think certain IEM with poor shielding act like an antenna and just add to the problem.
So far my best pairings are ER4S/IM04/IE800. They sound great with no need to add an amp.
M is a good source for wagnus however I dont feel the need to have it.
Hmmmm. I've actually had this happen when building an amp for FM radio in the lab. The cable connected to the power supply was picking up nearby radio signal, and we heard it out of the speaker. I have an old Sony DAP which back then had portable FM radio feature built in. Guess where the antenna for the radio was? On the damn headphone. The headphone cable was the antenna. I don't know exactly how they did it, but possibly stock headphone had a separate wire of specific length to match the FM wavelength for radio pickup.
Edited by SilverEars - 7/12/14 at 4:48pm