I'm a casual listener and I listens to my portable CD player mainly. Before, all I used was those $10 Sony Fontopia Earbud headphones. The sound is pretty good for the price. But I've decided to upgrade to a better headphone (for better sound quality) since music is a big part of my life.

I bought a Sennheiser HD 590 from the internet for $160. That's because I heard good reviews about it. I also tried it at my local Tweeter's and it sounded descent, but not as good as I was expecting. Maybe this is because the display model is not completely broken in yet (the Tweeter guy told me the headphones are used very little, only a couple of minutes everyday). Also, the source was a mere portable CD player.

The problem is, I've listened to some Koss headphones (supposed not as good as Sennheiser) at a music store and it sounded very good.

My question is, how much effect does the source (portable, home system...) have on the sound quality of a high end headphones.

Can a headphone amp improve sound quality? Or only makes it louder?