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.
 

Quote:

.................Anyways, just passing on this tidbit.  I can't guarantee all K81 drivers have 3 vents covered, but mine did.

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My K518 DJ headphones have 3/4 vents closed up with glue.  It is possible that the person who sold you the driver had previously tried modding it before they sold it to you.  Was that headfi'er  c****s****r by any chance? 
 
To be honest I feel the K518 headphones have enough bass as standard so I would close back up the three holes.  not remove the glue from the K81 driver.  blue tack mod the headphones aswell for a larger soundstage and reduced boominess if your interested. 
 
Here is a picture of my unmodded driver.
 

 
 

Interesting, no I'm told the drivers I bought were not modified. 

Hi guys...I´ve tried to fix one of my cables on my K81dj headphones and I´ve destroyed one of the speakers...where did you get this speakers? I´m from Argentina, and I´ve been very happy with this headphones since I got them...so now I still want to keep them and just fix it...I´ve been searching around ebay and Amazon if I can get this speakers...can you guys help me with this?
 
thanks!