I have the HD600 which are open back. If I put the headphones on my leg the acoustic sound of my strings drowns out the headphones.
Thanks Fro, that puts it into a perspective I can relate to. I'll be playing in a small closed office where I don't think the wife will be able to hear a thing. I don't usually play in there because it's very tight, but it will do I think. No getting around the string noise so if the open backs aren't as loud as that then I'm good with opens.