I second whoever recommended Stax, if you have patience and budget for a vintage amp+HP (~$600) - it doesn't get more transparent than that. They're also probably the closest thing I've heard to a full-size Etymotic.
K701/702 is a great headphone in the $250-300 range too, and I think will scratch your transparency itch. But... really keep an eye out for an opportunity to listen to a Stax Lambda rig, even if you don't buy one. It will rock your world.