[DoctaCosmos]

i found his page but no list for the 400.  also after reading his first paragraph i wonder how credible it is anyways.  but either way the decay and frequency response it's the same as time relationship.
 
  have you ever set up speakers in a room?  it's very easy to have an unstable stereo image.  reflections cause this to happen.  When frequencies bounce off the wall and hit your ears at different times then it breaks up the image and personally causes the sound to hurt my ears.  you have left and right. they need to hit at the same time and the less noise afterwards the better.

 

[DoctaCosmos]

but i see where you say there is no ringing.  i believe there is but because it's such a fast headphone it happens quicker and doesn't muddy the sound but rather can hurt your ears especially in the high frequencies. the graph shows there is

 

[TMRaven]

Yes I have a PSB rig.  Usually about the only time you hear people talk about phase with speakers is when they're out of phase due to inverse polarity, or when you're implementing a phase filter electronically.  
 
When there are certain frequencies with longer decay due to reflections and other reasons in a frequency response, we usually call that ringing.  The ringing of headphones as shown on Purrin's waterfall plots are almost always a component of bad acoustic damping.  Closed headphones can struggle in this in comparison to open headphones, because it's harder to deal with a smaller, closed cavity, but there are exemptions in either case.
 
How fast a headphone is has nothing to do with whether or not it rings or not either.  The HE-400 hurts your ears at higher frequencies simply because it has a lot of high frequencies.  They're well known to have an upper treble peak coloration.

 

[DoctaCosmos]

i was just using the speakers as a way to describe the affects of phase and imaging.  i know very well what ringing is as far as a speaker goes and i also know what intermodule distortion is and how you may be mistaking the two.a flat phase response means no ringing of time related noise. intermodule distortion will however show up on a wterfall graph

 

[TMRaven]

 
Anyways, I'd sell the HE-400 and keep the D600.