The 990 Pro should have some kind of fixed equalizer inside that boosts some frequencies and lowers others, so that you have the imagination of a much greater soundstage. (Damn psychoaccoustics ... it's so easy to fool your brain
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Maybe I try to open mine at the weekend and have a look if there is a little PCB with caps, resitors and coils inside or not. If thats the case, then you could probably remove it if you want the sound strictly in your head, or build your own to make the Consumer Class Headphone a Pro Headphone.
The Headband is a little different, but not to much. I guess it has to be a bit more confortable to sell it to normal HiFi Freaks, while for professional users it has to be as robust and easy to repair as possible. I think the comfort of the Pro is still ok. You can feel the clamping, but it is not to strong.
Edit:
I was looking at the description of the 990 Standard Version again, and noticed that they claim a "three dimensional soundstage" for that phone too. This must be new, or I was too blind to see it before. So I guess Mickey is right and both have the same driver and there is no extra PCB on the Pro. So I guess the whole thing is just clever marketing.
The audio NOOB does understand what three dimensional sound means, but has usually problems with technical terms. The pro user on the other hand does know the term diffusefield equalization and maybe doesnt want a consumer can, because they usually tend to fall appart in rough enviroment. So they sell the same headphone with only little optical changes and a description in other words to make most money out of both user groups
If that's the case, then I would recommend the cheaper one, unless you want to impress someone with a Pro phone