I ran both headphones with an Odac and a Schiit Magni for reference. Before I got either headphone though I was listening to my HD595.
The Mad Dog's sound signature suited my new found music preferences better. Maybe if this was a couple months back I would have preferred the HE-400 for much of my music. Bass was really important for me back then, I listened to a lot of hard rock and dubstep and all that. But over the past couple months, I went through a "transformation". Started listening to a lot of classical and indie rock and jazz tunes. I still listen to electronic, but no longer dubstep. I just don't like it as much anymore. I haven't even opened my itunes library in over a month, haha. Been listening to all my music on Tunes.io. Anyways... I just liked the Mad Dog's mids a whole lot more than the HE-400. I could tell why everyone says the HE-400 has a more "exciting" sound to it, but for my music I just loved the Mad Dog's sound.
My main gripe with the HE-400's mids: I'm used to listening to music at low volumes. Usually the only time I listen to music at higher volumes is when listening to electronic music. When listening to acoustic, jazz, classical... stuff like that, I felt like I had to turn up the volume on the HE-400 in order to actually enjoy the mids. Vocals sounded as good as on the HD595 to me, but everything else in the mids suffered. I'm not sure if this is the result of the drop in the upper mids or what, but I just couldn't get the same enjoyment in listening to those genres as I did with my HD595. So why didn't I just turn up the volume? Well I did, and the result... I don't know if it's just me or my tracks or what, but I still felt like I was missing a lot in the mids. And the more I turned up the volume, the more the bass from drums and low notes stuck out like a sore thumb. I guess the best way I could describe the HE-400 is that increasing the volume sounded like I was turning up the bass and treble more so than I was turning up the mids. Keep in mind this is on the Schiit Magni, and I'm not sure if this would be a problem with more powerful amplifiers.
Then I got the Mad Dog's, and as suspected the mids were better... much better. I could enjoy my mid-centric music again (yay!). My classical, acoustic and jazz tracks just sound so musical compared to the HE-400. Another thing I immediately noticed with the Mad Dog is it's detail. One thing I liked with the HE-400 is that, despite my issue with it's mids, I heard little details in songs that I never knew were there before with the HD595. Then I heard little details in the mids with the Mad Dogs that I never knew were there before with the HE-400. It just sounds to me like the detail retrieval on the Mad Dog's is better than the HE-400. I'm guessing it has something to do with the fact that the Mad Dog is closed back and the HE-400 is open backed.
As for the bass; I'll start off by saying that I definitely don't agree with some people saying the Mad Dog's bass is recessed. Like others have stated here, the bass sounds realistic to me. Compared to the HE-400, the bass sounds... I wanna say "much" less prominent, but I don't want to make it sound like the bass is non existent in comparison to the HE-400. My way of describing the bass on the Mad Dog: when a song has "boom" or "thump" I can hear that "boom" or "thump", it's just doesn't "hit me" as hard as it does with the HE-400. It's like the bass notes created by the HE-400 have a bit of vibration to their "thump", and the bass notes created by the Mad Dog "thump" but don't vibrate. Hope that makes sense. But keep in mind that's just referring to bass from drums and low notes on classical and jazz tracks. On some of my electronic music the Mad Dog surprised me. I remember someone in this thread saying the Mad Dog's bass sometimes surprises them and makes them smile, well that's exactly what happened to me. The Mad Dog is definitely capable of creating thumping bass, it just needs the right songs to do it.
The only thing that really made it hard to say goodbye to the HE-400 was it's open backed sound. In short, the Mad Dog sounds closed and the HE-400 sounded open (just my opinion people). Not to say the Mad Dog sounds congested to me, because it doesn't at all. I definitely feel a sense of space between instruments with the Mad Dog. But compared to the HE-400... you can tell which one is open and which is closed. But hey, at least I'll have a great headphone I can use on the go as well. And I have new found use for my Fiio E11.
Hope this helps.
