[gTen]

I am guessing you are going through the 3.5mm right? Have you tried using a miniusb->3.5mm converter from maybe a previous device? (make sure its an HTC one and not a regular one as I "think" the htc propitiatory miniusb has something extra for audio)

It may be possible since the 3.5mm was added non-standardly that it wasn't fully tested or implemented efficiently. Aka the could have just put in any headsets, tested it and said its ok just to get it out the door. Personally if you got a last minute request to add a 3.5mm port, I then doubt you'd go through the effort on testing it with high sensitivity headsets..

This is only an assumption on the possibility.