OII Headband Update:
After reading a bit through the Stax thread on HF, I saw I could also focus my bending efforts on the bend directly above the housing, not just on the arc as a whole. At first I tried to bend them so they were basically flat, basically making the headband a continuous arc. This left the bottoms of the earcups way to far off the side of my face. So I bent the arc back into its stock shape (I had it squeezed it in so the arc was longer), still I had problems with the bottom of the earcup. So I tried the reverse, and bend the bend above the housing back to its original position and then even further in. I then gave the arc a good squeeze so it wouldn't press against the top of my head (big head), and was delighted to find the headphone now grips my head very securely and comfortably, with the earcups hooking quite nicely underneath my ears. Feels very secure and very comfortable, and the sound is just fantastic.
Here's my theory: when the bass and treble sounds right, you've gotten your earpad rotation correct. When the midrange sounds magical and the soundstage coherent, you've gotten your headband arc properly bent for your head, or at least you are on the right path.