I recently bought the headphone bag for my hd598 and after i put it in for 2 weeks i pull it out it get dirty like this
You BLEACHED your Headphones?!?
Looks like the Dye was poorly fixed on the bag material. Try some White Petrol, the Stuff Dry Cleaners use, or similar Solvents. Start by using milder solvents, such as Rubbing Alcohol, and if that does not get it off, graduate to stronger stuff. Don't try something stupid like Paint Stripper though. The vapours from those will most likely Dissolve your Headphone's Diaphragms.
PROTIP - Wet a Q-Tip or cotton earbud with the solvent, and start working from the outside of the stain inwards to the center. This way, you can prevent the Stain from spreading. Use plenty of dry earbuds to soak up the solvent and the dye with it.
As Always, YMMV. Try this at your own Risk. Test the solvents out on a hidden corner of the material first to see if it leaves its own stain.