After quite a long time, I decided to purchase AKG K712 Pro for my very good friend... Primarily because:
1) Hifiman HE-500 is well-known for its poor comfort and my experience with LCD-2 was more than enough to skip the headphone even though FR-wise, these would be the very best for my friend who is focused on metal music. Also, I would have to purchase the HE-500 used (HE500 are like 925USD new here in Europe). Soundstage could be an issue for me as well but no as much for my friend.
2) Alpha dog does not have any measurement available yet and waiting period is very long. Also, these are closed and most probably not the very best in terms of comfort from what some people stated.
In the end, Ive got a good deal on completely new pair with full warranty for around 475USD which is pretty great here because K712 Pro officially goes for like 575USD here. I didnt want to import them since additional costs would basically make the final price equal or worse than those 475USD.
====================================
I am going to examine the headphone against my Sennheiser HD800 (unmodded)... Is there anyone interested in the comparison? I dont expect them to outperform the HD800 but still, if they are good enough, I could just sell my Senns and let my friend purchase another pair (though more expensive one).