I'm looking for a set of closed circumaural headphones with very good noise isolation (active cancellation or passive), I don't know where to start. They will be used primarily for air travel so noise isolation or cancellation is the primary concern. Some turboprop aircraft I often fly in have cabin noise levels well in excess of 90dB with most of the noise below 100Hz. I can't seem to find a good source of information on the noise isolation characteristics of various headphones.
Sound quality should be pretty good but does not need to be audiophile-worthy, mainly should have a relatively flat frequency response down to 20Hz, low distortion, clear highs.
Must also be comfortable for several hours of wearing.
As for portability... that isn't a concern at all. I don't want to compromise noise isolation, sound quality, or comfort for the sake of portability. I've got plenty of room in my backpack for a full set of cans.
Budget is $500 max.
Primary source will be my Cowon iAudio X5L but will occasionally use them with my laptop on longer flights (for movies). I listen to a wide variety of musical genres.
Thanks for the help.