I don't have the energy to do an in depth comparison and A/B test at the moment but I'll do my best of the top of my head.
First of all, the AD2000X has a very unique sound signature as it is very mid-centric while the PHVI has a more balanced sound signature, with bass a bit north of neutral. Bass quantity/quality is definitely better on the PHVI, which is expected since it's closed back. Soundstage is significantly wider and deeper on the PHVI, but the AD2000X has a taller soundstage. The PHVI presents the soundstage in a much more natural way, with much better layering and placement of instruments. Imaging is also much better on the PHVI. Each instrument and vocalist is very clearly defined in it's own 3D space while on the AD2000X they seem a bit diffuse in comparison. This is not to say imaging is bad on the AD2000X, but it's just that good on the PHVI. As expected, the AD2000X does have quite a bit more air, being an open back headphone. Overall, I would say that these two headphones are very different and cater to totally different tastes. In my opinion, the PHVI is technically superior in almost every way except in the height of the soundstage and the amount of air its sound has. It is several hundred US dollars more expensive though, so I guess that should be expected.
I'm running my PHVI directly out of my Sabre DAC while the AD2000X is out of the Sabre DAC and E12, just fyi.
If you have any aspects between the two headphones that you want me to compare in further detail, let me know.
I'm surprised someone told you that they're bright, as I don't find them to be bright at all. They have a warmer sound signature with a polite and non-fatiguing treble. I haven't heard an HD800, but that's pretty amazing if that's true. The BA certainly provides tons of detail without sounding thin or analytical.
Thanks so much for your time, really appreciate your comparison
I guess why some people think PHVI bright because in some mainstream recording it can produce sibilance. Especially if they tested it via ipod or iphone. PHVI is a smooth sounding headphone, but they're not that forgiving to bad recording. (still far more forgiving than HD800 or T1 though)