Wow, that seems really expensive for a headband replacement assembly.  I think I read one or two people say their 507 headbands broke on them soon upon receipt, too.  If the drivers are the same, 407 seems to have much better bang-for-buck.

Yes. And 507 pads are clearly better seal and comfort wise. You can get those from staxusa as well. Probably cheaper from ebay though.

I haven't tried replacing pads yet, but I am curious to go between the velour and leather pads on the HE6s since a lot of people say there's an appreciable difference between the two.

My general impressions when I tried switching pads were that the pleathers made them sound a bit darker and with with more bass impact. The trade-off was that you get an even smaller soundstage, and the HE6's soundstage width isn't very good to begin with. With my preferred amp (a Bada PH32 speaker amp), the HE6 doesn't sound too bright at all even with velours so I immediately went back to the velours and use the hifiman pleathers on my modded Q40.

With the pleathers, I felt that the diminished soundstage was because the pads were too thin. I think if you can buy two sets of pleathers and stack them on top of each other (like what a lot of T50rp modders do), then you can get the slightly darker sounding sound without the soundstage tradeoffs.

That would significantly change the sound signature too though, not just increase SS.  Generally speaking, you'd get moar treble and less bass.

Yeah that's probably true Now I'm thinking of those pad mods where you get slightly smaller pads for the T50, and then wrap bigger pads around them? I think the "bigger" pads were SRH840 pleathers on the T50.
Really should get Dyaems to talk about his modded T50RP, he's a member here.

Another thing I should compare is what's brighter between double stacked pleather pads and the stock velour ones, for the HE6.