I haven't had them side by side but I have had many of each opened up in front of me. The only difference I can see that could affect acoustics is the cups are made of different woods.
Yes, someone please do an actual side by side so we can settle this!
How would you determine they use the same exact driver? Is that from a visual inspection of the driver itself?
Haha, we need someone in the z7 thread who has both the D5k and the X00's. Wouldn't that be nice?