At the risk of being a copycat, I agree Earmax is very fine with the 600's. I found that the choice of IC's into the Earmax inputs could make big differences in feeling of reality (as defined by the headphone experience), and ability to convey emotion of the music, and it can serve as a quick screener of the quality of IC's before they are inserted into loudspeaker systems which use tube amps.
Headphone amps
I like to listen to music at night - my wife likes to sleep - I can't understand why she does this with so much good music to hear. Headphones have been a blessing for maintaining peace given our somewhat different nocturnal patterns. I'm soon getting a new integrated amp without a headphone jack, so I will require a little amp for the headphones. Please provide recommendations for headphone amps that you have used and loved. I like to listen to jazz and classical at night. I'm using a FMJ 23 as a source and I tend to like tube sound. I have Sennheiser HD 600 headphones.