I fried one of my AKG K81DJ drivers. A fellow head-fi'er here gave me a great price on a K518 driver. Aside from a post I found somewhere reportedly from an AKG rep which said the drivers are identical, I could not find other confirmation.
I took a photo of the two. From the drivers I had, the only difference was in the venting. The drivers appear to be identical, have the same number stamped on them, etc. There are four vent holes in the back of the driver. The stock K81 drivers had 3 of the 4 vents covered with a small piece of tape and hot glue. The K518 driver had all vents open. On the photo, the K81 driver is the left driver.
My K81 AKG headphones now have one K81 driver, one K518 driver. I removed all venting from the K81 driver. The result to my ears is more bass, more boomy, perhaps a bit more detail. It's hard to compare but very similar in the end. I personally prefer 3 of the vents covered. Maybe some would prefer 2 of the 4 vents covered. The problem with fine tuning is the ear pads don't hold up well to multiple installations and removals.
Anyways, just passing on this tidbit. I can't guarantee all K81 drivers have 3 vents covered, but mine did.