I would not call the DT880s neutral, although they're very likable. They are very sibilant headphones (at least in my opinion), but they are extremely lively. For a truly neutral headphone (and I know some will disagree) i'd go with the HD650s. I prefer them to any other headphone for referencing. I've compared them to the UE10s, K701s, DT770s 80 / 250, DT 880s, Proline 750, HFI 700, HD600, HD280, HD25, ER4, MDR 7509, MDR 7506, D5000, E530, Triple-fi Pro....I've yet to hear the XS-2 but theyre on their way.
Of all those mentioned I LOVE the HD650s.......some say they're not much different from the 600s but I think its a world of difference. HD600s are punchier and not as full, they suggest a little more highs (DT880s have even more highs but I don't think that's neutral), the 650s are a little more laid back but MUCH smoother and much more "musical"......I think the K701s are a bit thin, but very wide sounding.......I would say the K701s are the analytical equivalent of the more Musical HD650s.......very different sounding and will appeal to different listeners. I personally prefer the DT880s to the K701s, BUT again I will say to get HD650s even though you own the 600s