Can't help but notice Amir likes the DX3PRO
This little 200$ bugger puts some boutique gear to shame - including my uber fantastic Chord Mojo
Speaking of Deckard - it proves people will buy 1KW amp for a pair of headphones just to "have enough power / headroom".
I wish this review appeared on head-fi - some of the guys there would hyperventilate (not joking at all). I remember trying to argue something like Mojo has plenty of power to drive even the Audeze LCD-4 headphones (which I owned at that time)- to no avail!
I wouldn't say that having headroom in the power output makes no difference, but how much is really necessary being the question.
I have put my HD6XX and HE4XX in subjective listening comparisons using balanced output on my HPA-3B connected to my ES9038PRO DAC and my tiny little Sabaj Da3. Both connected to my laptop using Foobar and swapping between between WASAPI.
At volumes that were as loud as I'm comfortable with on both, they do sound a little different, especially in the lower regions of bass and mid-bass. Its highly subjective no doubt, but that's what my ears tell me at least. The treble areas were not that clear cut in difference
Will need to do a more objective, double blind test with the help of a friend to be absolutely sure. The possible areas outside of subjective placebo would include the DAC chip, Opamp implementation. And also the available power supply from the amps powering the headphone.
So its not completely clear yet how much headroom is "enough", but they can sound different even though different ones can supply enough power to volumes that are at the limits of comfort, at least in my own experience and experimentation. (As the Da3 has way, way less power than the HPA-3B. )