[fallingreason]

To make my question clearer, I am wondering what else plays a role in creating the sound signature of a headphone/IEM, as I believe it cannot just be the frequency response.  I've heard headphones and IEM's with supposedly very similar frequency responses but sound markedly different.
 
Any input?

 

[NotJeffBuckley]

I know next to nothing about IEMs. Or electrostatic headphones. Also not so sure about all this planar business. So, sticking to what I know...
 
For dynamic headphones... Impedance, driver mass, sensitivity and power handling, extension/reflex, impulse response, type of driver design (e.g. AKG/Sennheiser's Duofoil errr Varimotion, double the driver mass in the center causes it to act as a sort of partially rigid body semi-spherical radiator while the rest of the driver element is more flexible and reproduces the other frequencies - no need for a crossover, clever stuff, though it can still be a little phasey I think), type of enclosure design, specific enclosure qualities.
 
When someone says PRaT I literally think that string of stuff in my head.

 

[maverickronin]

Phase alignment, amount of distortion, spectral content of the distortion, internal reflections or lack thereof, the size and alignment of the transducer relative to your ears, and whole lot of other stuff.

 

[NotJeffBuckley]

In subjective terms, their cosmetics and how much you paid for them can affect how much you enjoy them, as well. File that bit under "psychological factors" I suppose.