Good response by slindeman.
I have low impedance, high sensitivity (Westone UM1) IEMs as well. I also posted in response to another 'audio hiss' thread in this forum, and suggested the Shure in-line attenuator to solve this problem. The hiss doesn't bother me enough to warrant me either getting an in-line attenuator, or building my own cmoy, so I've just "1. Listened to my Pocket PC with my high quality phones that hiss".
I would also like to refer you to
www.head-fi.org
It's my Aximsite for headphones =P