Hahah that was a funny moment, I think the OP did do the same comparison last night here on headfi few posts back.
Here's a review of HE-400 (compared to HE-500 and 300) on a Chinese audiophile forum:
The gist seems to be that:
HE-400 has fairly open mids, but they lack a little warmness.
treble is a little sparkly, good for energetic music like rock, but those who like very flat treble probably won't like it.
Dynamics are a little less than the HE-300.
Bass is nice and strong, however lacks a little tightness and impact (compared to HE-6 fully amped).
HE-500, in comparison, has perfect mids.
Both treble and bass are very "polite" and tame in comparison to HE-400, which could be a pro or a con.
His conclusion is that going from HE-300 to HE-400 is a complete shift in sound, while going from HE-400 to HE-500 is a maturation of sound; and that HE-400's sound signature is more aimed towards Western (our) audiophile tastes.
EDIT: wait that's written by OP in this thread haha (just noticed).