I've owned both the DT-880 Premium and the HE-400. I have never listened to the AKG 702 though so can't offer direct comparisons but will try to help. The DT-880 Premium (600 Ohm) had the least clamping force of any headphone that I've ever owned and some of the softest earpads. They were very comfortable. However, if you want to move around with them or be active, I can imagine they could potentially fall off. I did not move around with them on and had no such issue.
The HE-400 had more of a clamping force, but nothing that I would say was bad. They were, however, a bit heavier than your average headphone and many people find them uncomfortable. I personally thought they were okay, comfort-wise, definitely not great but also not horrible.
In my opinion, the most comfortable headphones that I've ever owned were the Sennheiser HD700. They have my favorite headband, extremely comfortable ear cups, and basically disappeared on my head when I put them on.
All that said, in my experience, comfort is extremely subjective so I strongly suggest trying before buying or at least buying from a place with a good return policy if possible in case they don't work out.
