In short... the headphones are AWESOME! I love them. The sound is good, but in reality the best part is how freaking comfy they are.
I did not buy an amp to go along with them, but I still have some Amazon bucks left so I am going back through your list and will pick one later today. The volume, though good, could be a little bit louder... and I suppose the amp could also add to the audio quality in general?