x2 on the Bifrost recommendation. I'm listening to this combo at the moment and 2 days in I haven't heard any glaring flaws. It's not the be all, there are deficiencies in lower bass weight and presentation compared to the NFB-10SE (particularly apparent through the LCD-2), but this may actually be a good thing for the HD650. It leans more towards mid bass punch than lower end flab.
I find the upper mids and treble section smooth and well extended. Details in upper regions are highlighted, but not to the point of exaggeration, and there are no hard edges to introduce listening fatigue. It's not the thin, cold and overly clinical dac I was expecting when reading between the lines of pre burn in impressions. I feel it's a "musical" sounding component, with the zest coming from nicely accentuated handclaps, snappy snares, guitar strums, vocal detail etc rather than explosive dynamics and bombastic low end rhythm.