So I own these, just got them with the sale, and I have to say I actually enjoy them. The sound was more spacious then I would have expected coming off my AKG712 Pro or my ATH-RX70, two headphones, which I use fairly frequently.
There is definitely and interesting element to the soundstage and imaging, probably a result of it being planar, but I feel for most people it will be a preference if you like it or not. Which I think ultimately makes or breaks this headphone for a lot of people.
That said, coming off the 400HE my first and only other product from Hifiman I feel like if you have a choice between this or the open back, you are probably should go with the regular Sundara unless you just really need or want a closed back.
Banana Phone by Raffi.