I bought just about all of beyerdynamic headphones (770s, 880, 990s and I got each one in 250ohm and 600ohm) just to test them, and I returned them all except the 880pros 250ohm.
I found the 770s were not very linear and added bass to everything, plus I didn't like the closed back. I like to be able to hear my tv and wife and kids when I am playing if they need to talk with me so I liked semi open back cans.
The 990s were also not as linear or natural sounding as the 880s, but they were better than the 770s.
The 880s were definitely the most linear and natural sounding of all of them. Then I had to decide between the 250ohm and 600ohm. I felt that the 600ohm had a significant reduction in volume that I didn't like. It did sound ever so slightly more hifi than the 250ohm, butnit literally might have just been in my head. Also, if I picked the strings lightly, there wasn't enough volume to make them audible after they began to decay. The 250ohm allowed me to play lightly and still give me enough volume to where I could alter my picking dynamics and still hear the notes as they decayed in a more natural manner.