I have a D2/A1 combo as well paired with some AKG K702s and some Klipsch WB-14s. CD quality 16 bit / 44.1 Khz audio is perfect for human hearing. If you are editing audio, it's definitely worthwhile to have a higher quality sample than that, but for listening purposes, you aren't going to benefit from higher than 16 bit / 44.1 Khz. It's not necessarily a bad thing that the D2 will decode higher samples, but that's not a selling point. That's not what you want to buy a DAC for. Unfortunately, most AV equipment tries to get sold by the numbers. You just want it to accurately convert a digital file to analog without screwing with it.
I highly recommend buying JRiver, set it to exclusive mode and WASAPI - Event Style, and you should be golden.
Your headphone amp and headphones will cause way more noticeable differences than the DAC (although, the DAC is definitely important). I've been really happy with the A1/D2 so far. I did have an issue with noise, but I contacted them and they told me to remove the ground pin on the UPC. Because I had it on the same circuit as a very large PC power supply unit it was causing issues. Ryan at Maverick Audio is really cool and helpful. I was super surprised at how easy it was to get some help.
