I use headphones to engineer CDs for release. I spent three days with my audio rig and the 590s testing the balance with my equalizer and reference recordings. It was stone flat from around 250k to around 9khz.
If you listen to rock music, you may not be aware (or even need to be aware) of what "flat" is. It's a common technique for rock engineers to boost the mids to make the music "louder" sounding. Just about every Rhino release I've ever heard has a massive midrange boost. Headphone manufacturers do the same thing to make their cans sound "more agressive" for folks who want to hear the guitar solos and vocals jump out of the mix. There's nothing wrong with that. Electronic music can sound any way you want it to sound. If you're used to very aggressive midrange, the 590s might sound like they have recessed mids to you, but that doesn't make it true.
There are a few people here who seem to listen to classical music on a regular basis. Those are the ones who are more apt to know the difference between "flat" and "colored". If you put on a well recorded Beethoven string quartet and listen for a bit, it's very easy to identify coloration. The balance between the cello, viola and violin is very precise.
Just because a lot of people say something, it doesn't make it true. When it comes to flatness, I would trust one opinion of someone who listens exclusively to acoustic music over a dozen from people who listen only to electric instruments.
See ya
Steve