It would help to have a stock D2 for comparison as I am unable to do an a/b comparison for the reasons discussed above.
Is the upgrade worth $400? My initial reaction is yes. I hear clear differences that I deem improvements, especially in the three areas described above. The bass slam and authority is the most noticeable. It’s like the D2 has a “dynamic” button that juiced up the overall presentation, but didn’t lose the essential goodness of the D2. The PS is very good quality too.
I will say that I think the switching PS in the stock unit isn’t bad. I loved the stock D2 out of the box, and the upgraded PS just improves on areas where I assumed were innate sonic attributes of the Wolfson DAC chipset (I have the same chipset in a portable A&K player and just thought the Wolfson chips were inherently mellow and tamer - adding the power supply reveals otherwise). After the upgrade, I felt the D2 moved a bit closer to the Chord Qutest in terms of overall dynamics, but without the Chord’s slight tendency to be a bit too dynamic in some instances. Hope that analogy makes sense.
In an ideal comparison I’d demo a stock D2 vs my upgraded D2 vs a stock T2. That would really demonstrate where the best value proposition is.