brian h There are many dedicated headphone amps in every price range and audio quality level. Do your homework and be an educated buyer but at the end of the day - audio quality is mostly "personal preference" and sometimes you'll buy/sell a few before you find your personal taste.

Good question; I decided on the HD600 over the 650 partially because of the 650's need for more power. These are very efficient headphones, and it's possible to drive them even with a small, portable amp, although not anywhere near their peak efficiency. I mostly use them with a dedicated headphone amplifier (the Little Dot MK IV SE tube amp) and they sound great. I'm sure I'd love the HD650 just as much if not more, however, I could only afford one so I went with the reviews and felt the 600's would be well suited for my ears and am very happy with the sound. Could I have done better? Who knows..., for the money these are an excellent choice. (If you require lots of bass you might consider the 650's, although the 600's give a very natural rendition using less power). I hope that helps..., good luck!!