So today I went to a large music equipment store to compare a bunch of different over-ear headphones in the sub €300 range, namely the: dt 770 pro (250ohm), dt 880 pro (250ohm), ath-m50x, k702, k612, k712 pro, hd600, hd650. Since I was trying to find a replacement for my old dt 770 pro, I wanted to compare these headphones against the dt 770 as it sounds out of my O2/ODAC, but unfortunately for me this store sells mostly studio equipment and they didn't really have any dedicated headphone amps there. But after some time trying random amplifiers I managed to find one where the dt 770 they had sounded very close to what I was used to. After that I was just going back and forth listening to headphones and (probably) annoying store personel for about 4 hours. For what it's worth I'll try to summarize my impression of each headphone in a few words:
Beyer DT 770 pro (250 ohm): Great bass, lacking midrange and detail, somewhat excessive treble (sibilant!), smallish soundstage
Beyer DT 880 pro (250 ohm): Nice soundstage and detail, less treble but still excessive (sibilant!), bass a bit too supressed. Overal better than the dt 770.
AKG K702: Unnaturally large soundstage, quite detailed, lacking bass, sibilant, somewhat boring.
AKG K612: Nice soundstage, lots of detail, decent bass. Frequency response seems quite flat. Much better than K702, yet still sounds a bit too thin?
AKG K712 PRO: Same qualities as K612, nicer bass response. Fun to headphone to listen to, would recommend. The orange accents look terrible tho'.
Audio Technica ath-m50x: Nice build quality, removable cable. Cheap sounding (considering the price), not sure why these are still so popular.
Sennheiser HD600: Nice soundstage, lots of detail, very nice bass response. Great to listen to, quite similar sound to the k712 PRO but slightly less treble.
Sennheiser HD650: Almost the same as HD600, but with a touch more bass and no weird marble finish.
I ended up leaving with the hd 650, since that seemed to fit my taste the best, and I'm really happy with it. The O2 also seems to cope with it well on low gain, on high gain it clips on some songs though, so I'll probably end up swapping the resistors to reduce the gain a bit.