I have to diverge a bit from my original statement.I don't think the HE500 is overpriced anymore. The HE400 can be improved a lot by mods, with the J$ pads that big suckout in the upper mid fills up a lot, but those pads were pretty pricey. I think in total, with shipping fees and import taxes I still payed about 80 euros for the, even though they were on 50% sale.
If you have the money, the HE500 is definitely better, just because it's almost flawless straight out of the box. Further pad mods make this headphone even better. I'm still very content with my HE400 though, the J$ pads fix the mid suckout, and a bit of eqing takes away the edge of the treble.
I wouldn't say HE500 is "perfect" straight out of the box, at most maybe "flawless".
It does not have glaring sonic issues, however a few aspects are lacklustre unless you do some mods or try new parts. Namely there is some hints of congestion, and overall clarity (especially bass clarity) is a bit meh.
Hence I went through all the trouble of modding the earpads / grills / trying out different cables to make them very close to ideal to my ears on all fronts.