My grandfather just purchased himself a very nice pair of headphones, the Beyerdynamic Tesla DT 1350 http://www.amazon.co_uk/dp/B004QMH8KI/?tag=tec053-21 He is currently using them on his Macbook Pro retina. He listens to good high quality music, and I feel he is greatly limited by the sound card in the mac. I am asking, what is a good, mid priced amp, probably USB, that is mac compatible? I found the Audioengine D1- http://www.pccasegear.com/index.php?main_page=product_info&cPath=211&products_id=19169 Would that be any good? Any others you would recommend?