First off i must say that i don't know anything about the headphone amp or dac. I had an expensive dac and after seeing a lot of proof that audio engineers on avsforum (a home theater audio website) about how digital to analog conversion is the easiest thing in the world these days to make perfect and basically companies are making a ton of money of them, i sold mine after doing very extensive comparison with my marantz receiver. So that's what i have to say about that. i'm going to get hell for saying that but oh well.
As far as the headphone goes, I've owned a lot of headphones and all have been dynamic driver headphones.
I almost decided to get another pair but told myself i was stupid for not trying the planar magnetic driver. It's superior technology in almost every way.
I decided to go for the he-400 and i can tell you that i have auditioned hundreds of speakers and have listened to headphones over the past 5-6 years daily. These blew me away. i lost my virginity to the whole "can't believe how much detail there is" a long long time ago but i have NEVER NEVER NEVER heard this amount and couldn't believe i have spent as much on audio as i had and never heard this. sorry for the run on sentences lol. Another thing is the speed of these. I never even understood what the hell attack and decay meant until i heard this headphone. I will never be able to appreciate a headphone that doesn't have this level of detail or speed . The detail extends all the way down to the bass.
As far as frequency response goes the highs are forward and great with some sibilance with poorly recorded material but not much. The highs draw attention to themselves at first but i feel some people listen differently. I always focus on the mids and once i get in the zone, everything else is basically ambient. What i like about these headphones is that when i get focused on the mids, the highs call attention to you and because they are good it only adds to the experience. Some people on the other hand focus on bass and that can be fun for sure. If that is you then i suggest looking for something with an elevated bass response.
Sometimes the mids are pretty recessed however. not all the times though. Some people will say certain genres sound better than others and while i understand that, i have found that it depends a lot on the recording.
Downfall that i have found, i have kids, they are loud. These things shine when you're able to focus on the music and the music alone other wise it's hard for your brain to go into music world and you feel like you're just listening to speakers attached to your ears. I only get time to caress my brain with these at night when i'm trying to fall asleep on the couch but when i do, i make little orgasmic noises because my brain is so pleased.
So because i listen to headphones a ridiculous amount, i'm going to try some higher end headphones that are closed like the audyssey closed back or the fostex th-900