I think it's best to avoid the angle pads; I can confirm though that the pads fit perfectly onto the stock ring and that they feel super comfortable to boot; I can also confirm that they tame the treble a bit and mitigate the upper mid-range recession too, which all sounds great on paper, but let me tell you, after three days of using those pads, I couldn't wait to get back to the velours. The angle pads, as nice as they are, seemed to make the mid-range sound a little boxed in and just a tad congested; also the sub-bass didn't sound quite as deep or clean to me, and not half as exciting as they did with the velours. I can only speak for myself, but I think angled pads do not work well on the he400; really, they create as many problems as they solve. I'm more convinced than ever: the pads just need to get out of the way. If the velours were more comfortable I wouldn't even think of upgrading, but unfortunately they do cause considerable discomfort when used for prolonged periods, and so an upgrade is a must.
Ooph, if it hurts the exceptional "openness" of the HE400 mids then that's definitely a no-go, even if it tunes the voicing a bit for the better. It seems like the only theoretical pad upgrade for these then would be velour non-angled pads that are deeper, wider, and made with better foam/velour fabric.