Upgraded to these from the HD600, which were my daily drivers for the last couple of years. I've only had them a couple of hours but I can say that the jump in overall quality is fairly stark. We all know that the HD600 have virtually perfect mids, and that still remains true. However, that is the only area where the HD600 is superior, and even in that regard, it's not by a lot at all.
Sub bass on these are orders of magnitude better (HD600, what sub bass!). Whilst they don't slam like a dynamic driver, the bass resolve and detail is the best I've ever heard in a headphone and they're extremely fast. Less mid bass than HD600, which leaves them a little more clinical sounding, but I actually prefer it.
Mids are more recessed than the HD600 and a little less natural, but again, the actual detail in the mids is top notch, and whilst timbre of the HD600 is unbeatable, these are not far behind.
Treble here is very very defined, detailed and well extended. You can actually taste and 'feel' hit hats and cymbals. I do sense a slight peak around the 6khz region, and this is one of the only areas of the headphone I would deem as clearly needing a little EQ. However, the highs are far from silibant or sharp, they are just slightly too hot.
Soundstage is night and day. Closing your eyes, it feels like being in a concert hall. So this is what wide soundstage sounds like huh...
Imaging and seperation is frankly insane. Laser like precision.
I will also mention that I have the Sundara, and whilst they're a very good headphone, they didn't quite dethrone my HD600. Low end was clearly better, as well as soundstage (somewhat) and imaging. But they didn't do quite enough to take the spot. I reached for EQ very quickly with the Sundara as I found them a little brittle and unresolving (audio is very subjective huh). Whilst I can hear that I can improve the Ananda with EQ, I don't feel in a huge rush to do so. Honestly, the FR of these headphones are very good indeed, and I would be quite content to leave them at stock tuning. But...I won't haha.
So, I have sacrificed a little midrange timbre (and I do mean a little) for vastly better low end, much more detailed treble, hugely better soundstage and very superior imaging.
I would very much consider these a direct upgrade from the HD600, and yes, quite superior to the Sundara.