ProtegeManiac
Headphoneus Supremus
How about something a bit more demanding down the road? Like the HD600 for an example. I plan on purchasing the Audio Engine D1 also.
Like I posted above, when I had the CD60, its built-in headphone amplifier sounded a lot better than the Little Dot MkII if we're talking about the HD600 specifically. Less bloat in the bass, clearer midrange, no appreciable difference in dynamics despite the bigger power supply on the tube amp.