But how would you know?
If you have to run something through something, ie can't be run alone... You can make inferences and assumptions based on lots of stuff but you'll never know.
I believe every headphone/earphones have their own sound signature, no matter what you run it from.
Signature will always be there, but perhaps the level of distortion differ from amp to amp.
But who am I to judge this? I've got no amp, little inventory of headphones and most importantly, a noob :P
Well, at least I got chicken (If you know what I'm saying :P)