Having owned and heard DT880 & 990s, both 600 ohms, I'll have to disagree as well. When ABing, I can't volume match the 990s to the 880s. I had to listen first on the 990s and then pot up 880s. Otherwise, the treble spike would drive and ice pick into my ears. And because I had to match the 990s where the spike doesn't hit as often, the 880s had "closer" mids and lower lows and highs. They just sounded better overall on the 880s and I didn't have to worry about that nasty icepick. In the end, 990s got sold off. The bass was definitely fun to listen to but I couldn't take the highs. There's entire albums that I had to avoid when I had them on.
Then there are obviously great differences between the 600 ohm and pro version. I've seen some websites score them completely differently as well. Doctorhead.ru gives the Pro version 9.9/10, while the 600 ohm version gets something like 9.2/10. Its possible the Pro version is more neutral, which is logical since its more tuned for studio use and has a completely different grill and enclosure compared to the Premium models, unlike the DT880 Pro's which are identical apart from the headband.