I added more dampening to the topmost ring of the cups. This resulted in even cleaner sound than before. Tone is at its "purest" now, if that's the way to put that. I tried covering up part of that ring to see how the bass is affected - so yes, covering it up will bring out the bass a little bit, but you should still leave one of the two larger holes uncovered so you do not get a closed-in sound. Well, painter's tape is definitely not the right stuff to use, so what is? I don't know. I'll try epoxy putty later.
I tried using the headphone with the topmost ring removed on one side. That resulted in a loss of control of the sound and
even more bass roll-off. So with that big wall in the middle of the ring, it seems like the headphone functions as both an open headphone and a closed one, so closing it up more will give more bass, just don't close it up completely. Partly closing it up will not result in mild reduction of the sound's openness or treble extension due to the cups also being open at the bottom. With most headphones, the cups connect to the back of the baffle (and create a seal, though some will not fully seal, leaving bass reflex(?) ports), but in this headphone, they do not. Instead, they stop a bit short of the baffle and give way to the perforated shell, which might be partly why there is bass roll-off.
Just covering up two of the four holes of the top ring resulted in the low bass beginning to appear, lol. The bass overall had mildly more presence.
Safety pins are a good way to remove the grill without scratching it. I should have done that the first times - instead, I used a pair of scissors like cotdt uses in
his recable thread and I scratched the grills as you can see below.