Impressions from my first outings:
1) Power
The efficiency of the cans sits something like 400s<HE4xx<400i in terms of power requirements. Running the HE4xx side-by-side with the 400i on my Lake People G103 seems to require an 1/8 rotation of the volume knob on the 4xx to reach volume parity with the 400i, while the 400s needs about 1/8 to 1/4 rotation less power than the 4xx.
2)Sound Sig
The first obvious results are a boomier bass (that seems to slightly bleed into mids) than either 400i or 400s, and more recessed vocals, by a good couple of steps. Not to say the 400i and 400s have less, just that it's more controlled, and thus less pronounced. This actually kind of saddens me, as I felt like vocals were top notch on the 400 series for the money.
the sound stage does sound a mite bigger on the 4xx. The microdetails are definitely more pronounced on the 400i and 400s, and the there's just a slight "obstruction" for the highs that makes it sound just the tiniest bit artificial and veiled. In terms of genre, these likely pair up better with instrumental music, or something where vocals are low key. Classical is surprisingly pleasant. French horns are present, but a bit muted, but most of the other instruments are pretty solid, strings are rocking. I just don't get the impact or intimacy that makes rock and punk so engaging that I normally get out of the 400x brethren. Metal fares a little better. Surprisingly, hip hop is alright.
I haven't had a chance to test this on my gaming rig, as finals are in full swing, so I can't speak to its imaging accuracy, or amp pairings. Currently, all impressions are from Modi 2U -> LP G103 -> 4XX. I'll run them through a Bimby->V200 and Bimby->MAC 1900 later to see if more power, and darker amp make much difference.
3)Comfort
I was actually really hesitant about the switch in headband, since I got in on Hifiman with the 400i, and 560, and only had experience with that headband, and had no issues with it. The new headband is comfortable, and the cables don't provide microphonic issues, which is a nice step up.
Overall, for the msrp, they sound damn good. I'm surprised they're pushing my g103 as hard as they are though. I think these would probably pair best with a brighter amp with more power.
As far as Massdrop specials go, I've got the TH-x00, T-x0, HE4xx, Noble X, and HD6XX.
If I had to rank them on pure performance, to my tastes, It'd probably look like HD6XX, THx00, HE4xx, Tx0, Noble X.
If value entered the equation though... HD6xx, HE4xx, Tx0, THX00, Noble X*.
The real difference between the HD6xx and HE4xx is a large matter of taste. The 4xx has more pronounced (meaning apparent) bass, and a different sound presentation. I'm usually more interested in microdetails and vocals, and the 6xx handles those things with aplomb for the money. The 6xx may have more extension in the bass, but I'm definitely getting the same impact from both, perhaps because the mids and highs are more present, its harder to volume match bass. There's a constant battle as to whether I want to wear the 400i or the HD6xx any given day, and that's with the HD800 and Ether's sitting in the background.
In terms of the the 400s, I still prefer the 400i and 400s, and if you can get those on sale, I'd highly recommend them. If you prefer soundstage to vocals, and a bit of boom to your bass instead of control, maybe the 4xx's are for you.
*It should be noted I'm just not into the Noble X's sound sig, as I'm a vocals man, and they're just too recessed for me, but the sound stage is killer for IEMs. Tradeoffs.