10 hours on my HE-400's now. Very pleased. I absolutely love the instrument separation. I'm shocked with the details. I'm hearing things I've never picked up before, on albums I've been listening to for decades. Comfort is better than expected - No issues at all. (W/Velours) The HE-400's have done well with everything I've listened to, other than they seem to expose bad recordings for what they are. My only concern right now is my other headphones might not be getting much use.
I'm a little surprised by the power needed to drive the HE-400's. I'm using the Nuforce Icon HD as the Dac/Amp combo. The HE-400's are hyped as "easy to drive" but I'm having to turn the volume pot slightly higher for the HE-400's, than I do for my HD-600's. They still sound great, I just wasn't expecting that. However, it's planted the seed that maybe they'd sound even better with a more powerful amp... Sometimes I hate this hobby. It's impossible to be completely satisfied. lol