The difference between 600 ohm vs 250 ohm is going to be pretty small, if anything.
This InnerFidelity article has some good information and impressions about that issue.
The DT880 is most certainly in the neutral camp, toward the brighter end. They don't add coloration in the mids or bass and are not euphonic in any sense. This doesn't mean that they're dull or uninteresting, unless one is looking for a specific signature. Since you've emphasized that you're looking for neutrality, they should be right up your street. As a consequence of their lack of overt coloration, the DT880 has excellent genre coverage and would make a great all arounder.
I searched for quite a while before I got my DT880 because I, like you, was ultimately looking for neutrality. I tried the V-shaped thing (M50) and the euphonic, colored sound thing (Grado), and while I liked the latter better than the former, it just didn't work well for all of my music. The DT880 did, and I've had it for almost three years now and have never regretted getting it.