[jaycee1]

There may be some validity to this. I have a pair of igrado's, which I ordered expressly for the reason that it is supposed to have slightly less treble than their other models. 
 
However, I thought upon first listen that my igrado's had a very noticeable bump in the upper midrange and treble. I thought this might be due to my amp. Therefore, I connected the igrado's directly to the player, and they still seem a bit hot in the higher frequencies. I can tame them easily by EQ-ing them, but it seems a bit suspicious that the grado model which supposedly has the least amount of treble emphasis still has a significant treble boost. 

 

[newbie1832]

Burn-in time difference?
 
then again the TF10 has BA drivers so maybe it's just defective.

 

[Jukeboxjonesy]

Okay, so I'm replying to this now as I just searched through posts relating to this because I have returned one pair of AKG K550s (due to a minor cosmetic issue) and the second pair sounds different!!  The first pair sounded very, very transpatrent with very subtle bass.  So subtle, that although I had read numerous reviews and was expecting light bass, I thought them a bit too light on the bass really but acceptable due to the unrivalled clarity.  
 
The second set are a different animal.  Exactly the same model, in a sealed box (as where the first pair) BUT... I now have bass!!! Admittedly, these aren't as run in as the first pair yet, but the bass isn't going to be less with more burn-in.  
 
There is a trade-off... the very slight naunces I'd never heard in my music before is still there with this second set, but not as pronounced.  For me, I'm happier with this second pair and glad I had the need to return the first!!  And before anyone suggests fitting on the ears or clamping pressure etc, they are exactly the same.
 
I can only assume it is all down to manufacturing tolerances and/or quality control.  Would be interested in hearing if anyone else has had a similar experience.  It would also explain some of the one and two star review on the K550 on Amazon... chances are, those people got a set similar to my first pair!