In my most humble opinion there is only one amp for the Sennheiser HD 600 headphone, and that amp would be the Wheatfield HA-2 tube amp, I am a lover of that glowing bottle. Have been using one in my system for well over a year. One must sample this amp to truly understand what it can do to headphones. I will say there is no substitute for the main system, but, when the requirements are that my house be quiet, I do use it with great success and infact look forward to the session. The Headroom people are the sole sellling agent of this amp. Good luck.......
