First, there are minor differences between those graphs that would slightly impact how they sound.
However, you are also misunderstanding how one can use frequency response graphs. Frequency response graphs don't tell one crap about things like transient response, compression, or distortion, all of which would affect bass sound quality.
Go look at the different DT880 measurements at Innerfidelity, learn how the other measurements function, and you'll understand how those headphones could sound different.
The minor differences are just that - minor. Minor enough that if you took 2 dt990 pro 250ohm and measured them, the differences would probably be just as minor as these.
Transient response, compression and distortion are all well and good but we are talking about the exact same drivers here, so bass sound quality would not change unless the cup design is radically different, which it isn't apart from minor cosmetic changes which do not affect the sound.
Also, I speak from the experience of having listened to both and compared them first hand. There are no differences in sound quality between the 250ohm and the 600ohm versions.
Here is a good topic if you're interested:
Edited by elmoe - 5/21/14 at 12:27pm