Smooth is definitely a word I would use to describe it - that and non-fatiguing, if I may use that word? It definitely allows you, the listener, to approach, rather than the other way around. It never forces itself onto the listener. It doesn't try to razor blade every sound at every moment. Rather it allows the listener to pick and choose which sounds to focus on, when and how the listener chooses.
I wonder, if I may add, that my combination of HD650 headphones with the smoothness of the Director DAC is why I find the combination inappropriate. The headphones are smooth themselves, which makes for my ears a lackluster combination? I heard the HD600 at a recent headphone meet in my metro area. The guy at my table had the identical setup - same headphone AMP and DAC, though he was using HD600. Late in the meet, I got a chance to compare. The HD600 are definitely a shaper sounding headphone.