I've been listening to a lot of vocal music lately, and switched back to the SR-Lambda for a comparison.
The HE-400, while not bad at all, also seems to have a similar issue to the SR-202 in that voices seem to sound...slightly textured/grainy, maybe? I can't quite find the words, but it makes the voices sound a bit artificial...or maybe I'm just really, really used to the SR-Lambda's presentation after having used it as my primary headphone for so long. Regardless, I still consider the old Lambda to be a superior headphone for vocals, even taking into account the mid-emphasized frequency response. No other headphone I've heard yet renders vocals quite like it does.
The best comparison I can think of, even though I haven't heard the other headphone in question, is how Mad Lust Envy compares the HE-400's mids to the HD650's. To paraphrase, the HE-400 has well-integrated mids with the whole spectrum that make vocals sound good, but the HD650 has this sweet, upfront presentation of vocals that puts the singer in the spotlight. (I have to audition the HD650 one day to make my own comparisons...)
I also can't shake this overall impression, even though it's been a while since I listened to one, that the current "numerical" Stax Lambdas (at least SR-202, maybe SR-303 too) are more like electrostatic HE-400s in overall presentation, just with bigger soundstage and slightly less tactile bass.
Also, it's now become apparent to me that both headphones can deliver proper punchy bass when it's called for, but the HE-400 will emphasize it more because of its frequency response.
Edited by NamelessPFG - 1/24/13 at 1:39pm