I don't think the frequency response charts really tell the entire story. Be careful when you see frequency response charts. I just take them as a grain of salt.
If you listen to the HD590 and HD600, they both sound very different from each other. The charts also don't represent many other characteristics about the headphones.
Unless you know which sound frequency really sounds like, its hard for anyone to correlate the frequency response with their heapdhones with the chart. You also have to remember that the chart is logarithmic in both directions. On the db scale, a difference of 10 db is like 10 times louder or quieter. As a rule of thumb, a difference of 3 db is twice as loud or twice as quiet. As for the x axis, the treble frequencies are squished and the ups and downs look much more volatile or exagerrated.
Also, the charts are normalized and smoothed. The normalized modifier means that headroom took a sample of several popular, but good, headphones and averaged them together. The normalized chart of HD590 is the difference between the HD590 and that supposed average. Also, smoothed means that they somehow got rid of the jagged spikes that actually exist in the raw data. They smoothed it because reading a chart with frequently occuring spikes all over the place can be hard.
If you are able to find the raw non-smoothed non-normalized data at headroom, you will find out that all of the headpones are far from a flat frequency response.