After reading your post and the german thread I tried this mod on my k701.
I have very good experience with adding bassports to Shure srh940 and also with adding an extra variable bassport to the Beyerdynamic dt770pro250ohm.
The bass in k701 goes very deep standard and is very clear, dry and articulated, but it lacks body and punch. This mod (removing the circular piece of tape on the back of the driver) adds some body and punch to the bass. Only the bass is affected so you will not loose any of those nice mids and clear highs, you just gain an overall more balanced sound signature.
I think the bass in k701 standard is overdamped.
With this mod the driver is able to move more freely without any unnecessary restraints but still sounding very good and detailed.
Some might think you loose something with this mod and yes, you do loose something, you loose some of that dry unrealistically light bass, that appears very detailled, but on the other hand you will gain the body in the bass, and body is essential for bassnotes to sound proper. The standard bass only appears detailled, but really it is not, it lacks the weight and body which in fact also is a part of the detail in the bass. The detail in the bass is not only on the sharp "edge" of the bass notes like a whisper, it's also "inside" the notes, in the body of the bass, the ressonance, that powerfull massive sound you can feel in your body at a concert or with speakers. This does not mean the modded bass sounds loose, slow or any less detailed or anything like that. The bass is still very fast, precise and detailed but now it also has the body and weight to back it up, just like it is supposed to have.
Akg must know how the k701 sounds with this bassport opened up. To start with they prepared the driver with this bassport, but they chose to shut it off. I simply disagree with akg in their choices regarding this, I think they went too far in dampening the driver. The bass just sounds better and more realistic when it is not overdamped, in my opinion.
Objectively it is not a matter af "better" or "worse" with this mod. Sure I believe I can make compelling arguments to doing this mod for gaining a better bass response in the k701, but really it all comes down to taste; do you want a bass with more body to it, or do you not want this kind of bass response?
So the answer to your question wether this mod actually works is: Yes.
Edited by Dafo - 7/2/13 at 1:19pm