Ok, the D3 arrived. I did a quick and dirty comparison with the E7 with the following setup:
Source was a laptop with Foobar2000, using WASAPI output. Headphones used were the Superlux 668B.
E7 was docked in the E9 and the USB cable was attached to the E9. WASAPI output and USB DAC, 16 bit output selected in Foobar2000.
D3 was hooked up to the laptop using an optical cable and connected to the E9 using an ordinary OFC RCA->3.5mm cable. WASAPI output, optical out, 16 bit output selected in Foobar2000.
I wasn't expecting that I could hear any differences at all, but I was surprised to actually hear some. First of all, the output level seemed a touch higher with the D3, but the sound was cleaner and less harsh in the upper frequencies when listening through the E7. I think sibilance was less of a problem with female vocals.
I won't have any problem using the D3, though, as the differences were pretty small (but discernible nevertheless) and I can confirm that it supports 24 bit output.
Anyway, what a surprise and eye-opener for me. I've always thought that DAC's don't really matter all that much, and I was almost certain I would not be able to tell the difference.