This was an amazing and fun purchase for me. It was fun to build and it makes my HD600s sound great. Very simple design that does everything I need it to for my headphones. I don't believe I'll ever purchase another amplifier unless I fall in love with a low impedance headphone that needs amplification.

Perfect headphones for most instruments and vocals, could be better for hard rock and metal. I've never been so immersed in music in my life than while listening to these. My Bottlehead Crack Amplifier definitely improved their energy and sound quality. I do find myself using treble boost on my ipod, however, and I feel like there could be more "fun" headphones out there. I will be searching for a more bass heavy headphone with a full bodied sound for my hard rock and bass-necessary tracks.