The reference is other headphones. For instance, despite its flaws, the Sundara sounds more precise for most instruments. Most sounds seem closer to me (subjectively) and more defined/vivid. On the HD800, by comparison, those same sounds have a very broad ghostly like presence. And this is something I noticed after months of extensively trying to fix the HD800 with EQ. As far as I can tell this has to do with the angled drivers which cause an exaggerated notch at 9 KHz. Here's a comparison with the Sundara:I wonder if you say that the imaging of the HD 800 is not correct, what is your reference? Have you been in the studio of the original recording or how do you know what is the correct location?
Sundara has a narrower notch and as result it does not sound as wide (oratory addresses the importance of this dip and its relationship to subjective spatial presentation aka "soundstage" in his Q&A) but on the plus side it does have a more vivid presentation (i.e. the sensation that "I am inside the music") as opposed to an exaggerated spacious sound where it is difficult to pinpoint the location of sounds subjectively. Of course, all of this is heavily dependent on your HRTF. This is just how I perceive it.
The other bad thing about the HD800 is that 11 KHz is too hot for me and causes sibilance but when I EQ-ed that out, the hollowness at 9 KHz became even more apparent and the spacious presentation was diminished somewhat.