I personally use Alpha Pads on my HE-500 and while I found them to improve the soundstage and imaging of the HE-500, it's at the expense of bass but you do get sweeter, more forward mids with a touch softer treble compared to the velours. They are extremely comfortable and they fit without so much of a stretch.
Though when I noticed that the bass has been tamed down, I did some modifications to the pads to lift the bottom end a bit.
I was a bit inspired by the Jerg/Modulor pads.
First of all, I cut off a tiny portion of the lip for easier mounting to the plastic rings.
Second, I reversed the pads so that the underside is exposed and I cut eight roughly equidistant holes on it.
The results aren't exactly pretty because I didn't have an exacto knife lying around so I used scissors.
After the mods, low end frequency is a bit more present without hurting the soundstage but keep in mind that this is for the HE-500. I have no idea for the HE-400 though and again, I stopped at eight holes but I don't know what the effects will be if I add more. I will stay at eight for the meantime.