The DAC built into speakers like the A2+ and HD6 is solid and really helps make sure you get a consistent experience.  With that said, DACs like the D1 would bring you extra features like a dedicated headphone amp or easily accessible volume knob. In the case of the HD6s, this could also free up the optical input to use with another source, like a television, if you want. If you're considering the D1 with your A2+s, the D1 would be considered an upgrade from the A2+s DAC.