I am really impressed by the Burson Conductor Virtuoso headphone amp USB DAC and its sound signature: it is transparent, energetic, smooth and spacious. I find the rich and airy presentation, with superb imaging and timbre, to be a great match for the genres that I mainly listen to (film music, classical, jazz). I use it with the Grado PS1000 and BeyerDynamic DT990 and have it connected to my MacBook Pro via USB.
With the PS1000, the sound is dynamic, crisp and highly detailed, with an amazing, deep soundstage (though it is quite bright, so I have the EQ levels adjusted). With the DT990, the sound is warm, intimate and very enjoyable, too. Burson Virtuoso is an exciting, tonally accurate and musically engaging headphone amp USB DAC!