I believe now that my experience was due to the headphone extension cord I was using with the IEMs. They are indeed more sensitive than the Senns. That might account for the hum I hear, which is quite soft, but I hadn't noticed it before. I don't know if it is the transformer or tubes. When I tap on the transformer or metal base it changes slightly to a different frequency. It is not bothersome when music is playing, so no worries. I was just curious what others experience was with IEMs. My Senn 600 are the main headphone. I appreciate your quick reply, Paul. And, you have great products. I am trying to think of ways to build and use other products. Not a "need", just a "want"! Cheers!