Wow, quite lengthy discussion we have going on here and can't say I am disappointed because the stuff you mention is very interesting.
I am willing to bump up my budget if there are headphones out there that more than suit my needs then I'll pay it. However, if like some you say, are only marginally better then there really little point in me forking out the extra.
I don't think I will go with the NS1000, I just don't see the point. If I want to change headphones I want them to be a lot better and from what I've read about the NS1000, I don't think they will suit my needs.
I have considered the Sennheiser range again (did in the past when was buying my Goldring DR150) and always hear good things about their HD555 and PC360 but again, I am not sure how much better they are than my DR150's.
I don't know what is different with the Icemat Siberias, other than frequency response, but I find the audio to be overall much more detailed but without compromising the quality (which I tend to find in a lot in most gaming headsets with good directional sound but sound awful for music and movies). With my Siberias I can listen to music, movies and play games and they are all really good. I refuse to believe, that a headset like this cannot be bettered in that same price range. I have also tried others, for example I bought a Sennheiser PC161 headset 6 months ago and that was just an absolutely awful, awful decision - I don't know how people can say that sounds good in movies, music and games - it just sounded totally weak and flat in everything.
Edit:
Looks like we may have a winner.
http://forums.overclockers.co.uk/showthread.php?t=18240484