I got the chance this weekend to compare the sundara to the HEKse and I must say that I was really impressed with the sundara's stock tuning, out of the box, this is a headphone I can listen too without any EQ, the HEKse I can also do but less so, because it's so much energetic in the treble, not peaky but definitively the detail is brought forward more so, I feel I will get fatigue more quickly with the HEKse than with the sundara, BUT the sundara does have a peak in the treble that only comes with certain recordings and with certain female vocalists from time to time, but it's not so frequent that it becomes an issue for me.
Frankly, I have been really impressed and surprised with the SQ of Hifiman's offerings throughout their entire line up, now I can see why so many people love the sundara, the only criticism I have with the sundara are the ergos, it's comfortable enough but the fact that there's even less cup swivel even compared to a focal headphone does lead to a kind of clampy feel with the headphone, not anything near as bad as something as an HD6X0 out of the box clamp force by any means.
The HEKse is definitively more clear and detailed than the Sundara and it just run circles around the sundara with the spacial presentation and sound stage, the HEKse it's more extended both in the low and high end, it's more sparkly, but the Sundara does feel a little bit more punchy in the bass and I think I enjoy more the vocals on the sundara than the HEKse eventhough the sundaras are less clear and detailed.