I had extensive listening both with the D1 and the D2 with an Audiophilleo AP2 + PurePower USB-SPDIF converter.
The USB inputs on the D1 and D2 are worlds apart, with the D2 being far more refined and detailed. As a result of clarity, the D2's bass seems weaker, but it is, in fact, cleaner. The AP on the D2 makes a shocking impression. AP+D2 vs. D2 alone is almost the same level of difference as the D2 vs. the D1. Detail I have never heard before makes itself present. Switching the resolution to 24/192 further enhances this.
This would mean that the U2 USB-SPDIF converter that's built into the D2, while pretty good, isn't necessarily the best implementation of asynchronous USB out there.
However, on the SPDIF inputs alone, the D1 and D2 came quite close. Meaning that an AP+D1 sounded very close to an AP+D2.