You could try ordering a spare DT770Pro-80 cable from Beyerdynamic. The Premium models also use the straight cable.
What you do then is desolder the old cable, pull out the metal fork that locks the cable in place in the cup, pull out the cable, push in the new cable, push the metal fork in again, then solder the new cable's connections where they go. Easy as pie.
Alternatively, you can make like the modders here and go with some custom-made cable made from Mogami wires, OCC copper, etc. Google Image Search DT990 recable for examples.