I've been very impressed with my 0404 USB > Heed Canamp > HD650 setup. Great sound in all areas -- treble, mid and bass; soundstage; detail and layering; speed; etc. And I can't dial the volume past 9:30 or 10:00 before it's too loud.
Wasn't thrilled with the DAC/amp combo with my DT-990-250ohm. It really enhanced the already high treble to an uncomfortable level.
Neither the 0404 headphone output alone or Heed sourced with an iPod was as potent as the whole grouping. It does the job, but reaches its ceiling quickly and obviously.
I'm not sure if it is the Heed or the 0404 or the combo of the two that results in the high highs with my 990s. Through my iPod and Headfive, the 990s are smoother. I have not taken the time to isolate the specific pros/cons of each component, choosing instead to take the iPod/H5/990 to work, and leaving the Mac/0404/Heed/650 at home.