If you are looking for an extremely well built linear pair of closed headphones I cannot recommend the Sony MDR-7506 more strongly especially because they can be had for under $100.
If you are interested in their frequency response and waterfall here is a link (these were taken by a friend of mine and are accurate from what I can here):
http://forums.audioholics.com/forums...52&postcount=6
You will notice a slight dip in the response around 200Hz which clearly isn't perfect, but for the money these are great especially when you consider the fact that these headphones have little audible resonance - if any.