You've just discovered what I've known all along:
A Supra-aural headphone configuration can greatly change the sound characteristics of a headphone often times for the better. I believe that the Supra-aural design plays a huge roll in what makes up the Grado 'magic'. It is my theory that the way the parts of your ears interact with the pads/earcups with a Supra-aural design can add a nice visceral and sometimes even tactile Bass impact, and slightly forward mids. Supra-aural phones tend to have a 'loudspeaker like' presentation, at least to my ears. And of corse, the closer the drivers are to your ears, the more the quantity of Bass will increase, and the highs will decrease. To contrast, I have yet to hear a pair of Circum-aural phones that can do what a pair of Grados can do. And believe me, I've tried quite a few because I am searching for such a pair of phones. Please, if anyone knows of such a pair of Circum-aural phones - share with us.
I guess it is no suprise then that the KOSS KSC-75s are of Supra-aural design, as are the AKG K81DJs, as well as the Grado phones (with one acception) - all hugely popular phones among the Head-Fi community. Now that's not to say that there aren't a bunch of Circum-aural phones that aren't popular among the Head-Fi community. It is just that circum-aural phones and supra-aural phones are different animals.
To further prove my point, here is a little experiment to try if you doubt my theory.
If you are a Grado fan, and haven't yet tried a pair of GS-1000s with the default Bagel pads, do so. The 'Grado magic' goes right out the window - at least it did for me with rock/metal music. Now swap in a pair of Bowl pads - boom the Magic is back. Also, you can run a similar experiment with any of the other Grado phones, just remove the bowl or flat pads, and swap in a pair of Headphile C-Pads. Sure they are more comfortable, but the sacrifice is that the Grado 'magic' goes bye bye, or at least it does for me.
So my advice is:
If you are going for a Pad modification of some sort, make sure it is of a Supra-aural design. It also seems like you want the Supra-aural design to place the drivers close to your ears. Sounds to me like you need something similar to Grado Flat pads, and you would be all set! SInce the Ultrasone cups are designed as Circum-aural and are large, this may prove a difficult task to complete. So as you mentioned, it may require some sort of custom work.
Anyway, that is my theory on Supra-aural phones versus Circum-aural phones, and I'm sticking with it!