I haven't read through this whole thread, so this may have mentioned before, but I have a mod that I find creates a very good seal and increases the bass level, which I personally found lacking. First, I found the headband length that fits best (for me that is 1), and then I just bent the headband where the arm connects to the headband. When I say bent, I mean really bent:
The headband seems very flexible, strong, and resilient, so this doesn't seem to do any damage (still, do this at your own risk). Plus, the top of the headband is still round and remains very comfortable.
I think this is different from the other handband bending mod since I'm not squeezing the cups together. Instead, I'm angling the arms in to create a tighter clamp. Since the ear-cups swivel in and out the cups remain flush against your head after you bend the arms. I don't have a tiny head, but it's still pretty small (my fitted hat size is 7 1/4) and this really works for me. After doing this I can finally hear (and feel!) my K550 get down to 12Hz with songs like Kendrick Lamar's "m.A.A.d. City". Plus, every other positive attribute (soundstage, clarity, resolution, etc...) remains in place.
I was on the fence about keeping these, but this mod, combined with the E17 and its EQ, really makes this headphone a keeper. Of course, I can't end without saying YMMV