It's fair for people to see that the FR on the two are similar, as they are here and there, but with that said, the real-life experience sort of makes the graph absolutely useless. I never trust graphs as I only trust my ears, but sometimes I agree with graphs and the graphs back-up my thoughts. But with that said, take a look at the HE-400 graph. To me, it does not sound like what it shows on the graph. That's why I don't trust machines and devices to tell me the story as I'm a storytelling type of person and choose to do things my way.
Yes, the DT880 definitely sounds leaner while the HE-500 sounds much, much more full, liquid, smooth, and organic. But I won't be much help to the OP if he/she is considering the DT880 as I don't like a lot of Beyerdynamic products myself.
Yeah, again, they only help to explain stuff. In this case, they kind of don't, because they do not sound (to me) similar at all. Different measuring methods make stuff harder for us as well. Too much skepticism on them though, which are totally within the rights of the people, but I wish they would keep a more open mind at times, and be less argumentative.
I remember you share my hatred for the DT990, and love for the COPs. I'd tell the OP to go for the HE-500.