From what I've read or seen on reviews, I get that an amplifier doesn't (and shouldn't) affect the tonal balance of a pair of headphones. But I do read that it affects technicalities like instrument separation, and I hear the term "dynamic" being used a lot. Could you explain what an amplifier...

I'm looking for a neutral sound signature with good instrument separation and detail, with the caveat that the headphone also has light weight since I have neck issues with wearing headphones too long. These two are currently the best two I've found on my research, but I'm open to suggestions...