Well on one of the earcups on the HD600, if I look closely through the outer black grille I can see a single strand of thin bare bronze wire running from the bottom to the transducer... Is this a problem?
This may be aproblem.
You should open the mesh cover (It's so easy) and check what the wire you can see is.
The wires from connecter to transducer are normally covered by black plastic frame. If you really see the wire from outside, it means the wireing was not done properly. This may cause a problem. You can claim it to Sennheiser.
The wire may be just a waste wire, a short wire which does not have any electrical connection and just exists in the housing. Then you only need to take it out. In my HD600, actually there was such a short wire in the housing.
Don't forget, they were made in Ireland, not made in Germany. (Sorry Irish readers, I love your music and beer.)
After reading your post I checked my Senns: can't find anything inside like you describe in your cans. Try contacting Sennheiser thru email by using their website. Email their tech support, and maybe even include a photo so they can see what you are talking about for themselves!
Hope this helps!!