I followed Ari's advice to open up the headphones and take out the driver. I take off the grills and see that these have already been messed with (I got these from Deepak and assumed he bought them used), sigh. Someone removed the foam and the drivers weren't seated correctly. Sigh.
I take out the left driver and try to careful get off some hair/gunk. Then I look at the right driver and it is also not seated correctly, so much that when I barely touch it, it falls on the floor. I pick it up and see that it has a little crease on the driver. I have no clue if it was there before.
Oh well, that doesn't look too bad. I put back everything and plug it in and the right side sounds ok and the left side sounds much better but there is still a bit of distortion at times that is bugging me. So I keep opening it up, "carefully" getting more gunk off. However, I must have pressed my luck. The last time, I had trouble getting the driver out and once it came loose, my finger hit it severely and now the driver is really fucked (there is sound, but it is totally unlistenable). Sigh.
I know that I can get a new HD600 driver from Sennheiser ($50 each?) but I would probably want to get another driver for the right side too. Since these have been messed with and are old/used, I probably will just get a shiny new pair of HD600 from Justin instead. That is, unless there is a better mid-fi headphone out there now.
For now, I am using hubby's HD555 which are ok but not as nice as the HD580/600.