Your source will determine soundstage, whereas the k701 has the greater potential, these Beyers are only slightly less so and nearly indistinguishable in their ability at wide soundstaging when scaling up.
Of course both are highly resolving instruments which will present the qualitys(both good & bad) of the material played revealing the quality of the source used.
Between these two, the k701 is the more forgiving of less than "better" sources and the material recorded.
For this reason the 990s are difficult to recommend, due to associated gears and material commonly used by all to many, myself included until recently when I upgraded my source. With lesser or average sources they can present (due to their extended high freq extention) unflattering treble overly excieted. This is also true of the K701s albeit less so, and therefore somewhat more tolerant of average sources.IME
The 990s are less of a referance headphone but they are highly detailed in their mids, with the better bass response and the more "Fun" choice, which scales up extreamly well.
With the choice you are deciding upon based upon application & cost valuation, I would recommend the $200 DT-990s('05e) and put the other 200- 250 into a DAC to match with our HEED... That would be an exceptional way to build your system IMO!
Perhaps the iPod you reflected in your profile, might roll off the high frequencys fed your Heed and then the 990s and it may very well serve as a good source in that regard in that system... However, only by testing would you know how well a particular CDP would match these revealing instruments.
The safer choice between these two exceptional headphones would be the k701s as they are relatively less bright although highly detailed, more neutral, mellow and relatively warm in their relaxed presentation and are therefore my referance headphone. With less of an excietable treble although, which also will reveal sins of the sources it is fed, while having a more polite lower frequency (bass) presentation, they could be said to be the safer choice.
They both can do Classic Rock although, I prefer my 990s with this gerere, equally well the other generes you propose, with the preferance to the k701 for Classical, and both do match nicely to the Heed CanAmp...
Hope this helps in your decisions,
Good luck~
