Well, a lot has been said about DT990. (Over bright, too much sibilance, recessed mids, un natural bass response, etc.)
I think this is a misunderstood product. I find them really good for well recorded material. Their lower mids are really textured, and their bass is fast and full but don't bleed into the mids. Their highs are really detailed and have life-like energy and transparency.
Mstislav Rostropóvich Bach Cello Suites, Shostakovich Jazz Suites, and Vivaldi Four Seasons (Telarc) always shine through my DT990.
Anyway I understand why a lot of people tend to say what they say. If we pair this headphones with poorly recorded/over bright compressed music with low dynamic range (most modern recordings - affected by 'loudness war') this headphones can sound over bright and not engaging at all.
People also say that this headphones goes well with techno. I'm not completely agree with those claims. Many people who like techno (not everyone, of course) tend to decide if a headphone fit the genre based on it's bass presence only. Then they like DT990 bass and claim this headphones goes well with techno (which is not well recorded most of the times). And then, the same people who decided to get DT990 to listen to Mike Sheridan - Med Sma Skridt or David Guetta - Titanium claim DT990 are over bright and have sibilance issues.
Graphs are only graphs and often don't have much to do with how sound is subjectively perceived by us, but they are also pretty objective information that may help to get an overall idea about sound signatures.
I'm not saying DT990 are on pair with HD800 in terms of sound quality, but both are bright, both aim for transparency and good separation, and both are un-forgiving with poorly recorded music.
Most HD800 owners are knowledgeable audiophiles who have spent a lot of money on their sets and music, then tend to say: great headphones for classical, jazz and acoustic music and also sound good with well mastered material of other genres.
On the other hand DT990 Pro is quite cheap nowadays, and many non audiophile people get them (same as some akg k7xx) and use them with poorly recorded music (genres) and often under-amped and then they share their experiences.
Same happens with my Focal Chorus speakers, simply amazing transparency and detail at its price point, but playing poorly recorded material is also a terrible experience.