The DAC built into speakers like the A2+ and HD6 is solid and makes sure you get a consistent experience. With that said, DACs like the D1 would bring you extra features like a dedicated headphone amp and 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, it would be considered an upgrade from the A2+s DAC.
Would using an external DAC, like your D1, be worth using with your speakers with built-in DACs, like the A2+s or HD6s?
