Hi guys, While I love my current HD428s, I am looking into buying a pair of open backed headphones. I also would like to buy a (somewhat) portable amp to power them and my HD428s. My top 3 options for my budget are the Grado SR60i's and SR 80i's or the Sennheiser HD 558's. I mainly listen to Rock, Alternative with a good amount of House, Trance and Drum and Bass mixed in. Thanks for the help.