tried a (slightly used) external DAC (from a highly recognizable and reputable firm) in comparison to the outlaw rr2150 USB port. after a few minutes back and forth, I had to conclude that an external DAC is not worth buying for me - I will stick to the built-in USB port on the amp, which sounds much better (presumably because it means fewer cables are all in all). the external DAC was fine on the headphones in my office, had more detail and more oomph than the little portable headphone amp I also own - but at home, the built-in USB port on the outlaw rr2150 performs really well!