A further update:
Tonight I experimented with the headband's ear cup height adjustment, as I found that I was losing a lot of bass when I turned my head left or right, or when I opened my mouth or flexed my jaw, etc. I previously had the height set so that the top inside edge of the pad was almost touching the top of my pinnae, but now I have it set so that the bottom inside edge of the pad is almost touching the bottom of my lobe.
Raising the height of the ear cups has had a huge effect on the headphone's tonal balance, genuinely huge. The lower mids and upper bass now sound fuller, and the mid-to-low bass not only has more slam but also much more richness and warmth. In fact I can no longer hear a dip in the upper bass. The treble seems to have reduced / rolled-off some as well. I'm not sure if the treble reduction is an actual reduction or if I'm just getting this impression because the bass and lower mids are now louder. Also, now when I flex my jaw or turn my head to the side, the reduction in bass is much less noticeable than it was before. This all suggests that I wasn't achieving a perfect seal with the earcups at their previous lower height. It's hard to believe given how thick and plush the ear pads are, but I guess there's no accounting for listeners with narrow, bony heads and scrawny necks!
.
I'm curious to know what height other A2C users have their earcups set at. Do you have it centred so that the pad sits over your ear with equidistant spacing all around it, or do you have it set higher or lower?