Just making the announcement that, after several weeks, I put the modded pleathers (no spacer rings) away and am back to the velours. Some comments:
Spacer Rings - definitely something I hate about them. Pleather much more comfy without them. Either the seal quality, or the distance of driver to ears seems to impact sound quality somewhat - I consider it better without spacers than with. Something warm, fat, and nice about modded pleathers, but resonance problems live on, particularly when I've been listening to piano music. Fatigue sets in, and my ears get hot.
Back to Velours - 'wow these are kinda stiff', and definitely putting more distance between ears and drivers. Yet here that is not a SQ issue but more of an ergonomic one. Still soft and cool on the ears though. Not sure how good the seal is, but they definitely cup my ears in a different way. The ringing in the pleathers (even after mod) just doesn't seem to be present with the velours. Bass response and reach seems about the same, but maybe a tad less weight and, as noted, 'dryer' in presentation. Vocals actually seem more prominent to my ears, and yet more relaxed at the same time. Resonance is not a problem, and more detail may be the case because they do in fact, measure more treble configured this way.
Relaxed is the best word immediately coming to mind - more detailed and more spacious at the same time. The pleathers sound great, too, but a bit choked by comparison - performance has been left on the table.
Not that anyone cares about my personal flip-flopping, but if were chief engineer for Hifiman, I would probably conclude that the stock velours are really the way to go with these. The pleathers seem a little fatter and 'warmer', but they introduce resonance problems in the midrange that make the HE-400s sound more congested than they really are with some music. I can only speculate that the memory foam is too dense, or that there needs to be more of it and a better shape.
Maybe when they are released, someone can put some 400i pads on these and evaluate them - with frequency response charts and waterfalls . . . (Jerg, Purrin, Tyll . . . . . ). SCIENCE!