[Ancipital]

  This thread should be interesting to watch. I had to dig up an older thread with basically the same question. If I may add an additional question: what would be a good DAC/Amp that will drive the HE-400 and possibly the HE-500 and/or HD-650. Would you need separate amps for those?

 
I just did a quick google, and it looks like the HE-400 has the same impedance and sensitivity as the HE-400i, which I own. Handily, I also own the HD650.
 
What I can say, from first hand experience, is that the HE-400i is really easy to drive, but that the HD650 needs plenty of power. It's easy to get lots of volume out of the 650, but you won't get much bass or detail unless you're throwing a ton of power at them- more than you'll typically get out of a portable amp.
 
The HE-400i sound glorious straight out of my Chord Mojo, it can drive them in its sleep. A lot of portable amps would probably do a good job, as the HE400i are a cinch to drive (..and weirdly comfy, and amazingly clear- a bit of a winner for the price!). However, my HD650, not so much- they sound comparatively flat and uninteresting out of my mobile amps.
 
As an inexpensive stopgap, I grabbed a Magni 2 Uber- this really does make a difference, the HD650 suddenly has more pronounced and defined bass, and an increase detail in general- with the amp in high impedance mode. Something something impedance curve, something something voltage swing- analogue electronics aren't my thing, but the subjective difference is stark, and I can certainly pick it out blind every time. The 650 will generally benefit massively from a powerful, good-quality desktop amp.  I'm horribly tempted to pick up the new Schiit budget balanced amp, in fact (the HD650 can be recabled for balanced very easily), if only to draw on bags of power.
 
Essentially, pretty much anything will drive HE-400 adequately (assuming it's similar to drive to the 400i), assuming its output impedance is comfortably below 4.3 Ohms. An amp that can power the HD650, but that maybe has the ability to switch the gain down for lower impedance cans would do the job- it might be more efficient to get one nice amp rather than two cheap ones?
 
(The DAC question is easier- pick a nice DAC, depending on your budget, and how sure you are that you'll hear a difference. The Parasound ZDAC has a fanatical following, and a lot of folks really like the Schiit multibit stuff at or below that price range. There are lots of other good options that cost less, though.)