My impressions...
Got my HE-400 in the 2nd batch about 3 weeks ago. I connected them straight to my Audigy 2. I must say I was very disappointed in the first week especially with the vocals, they seemed to vary up and down in volume. I wasn't sure if I should keep them or not, although they were improving with more use. I never had any headphones that cost more than $50 before, so wasn't sure how to judge these.
I decided to go and audition some more expensive models. Had a listen to the HD650, HD800 and LCD2 connected to a Burson HA-160D. There was only about 10% difference between the sound of all of these but I liked the LCD2 best. They weren't that much better than the HE-400 though, not worth 3 or 4 times the price that's for sure. I found an old Pioneer vintage amp that is about 20 years old and tried that. The difference was night and day, not so much in the quality of the sound but there was no variation in the vocals and they sounded more smooth and balanced. They had good volume running from the Audigy 2 but the volume was on max, these need a good amp I think.
The bass on the HE-400 is kickass, even better than my short listen of the LCD2 I would say. They are great for movies too.
Running with this setup for the past 2 weeks, I find them really satisfying and enjoyable now.
I think I would be hard pressed to find much better for $350 (or $399 now), so I will be keeping them!!!