I just got my pair of RE-400s earlier today, and I have quite a few thoughts on them.
First of all, I have been using a pair of Hifiman HE-500s for daily at-home listening, and Hifiman RE-0s for on-the-go; but since the latter broke, I was in the market for a new pair of IEMs.
Coming from the RE-0, the first thing I noticed is that the RE-400 have a much deeper fit with the stock biflanges. This is great, because I was never able to get a good seal with the RE-0 and its shallow insertion depth. This in turn makes for much better isolation and fit.
The soundstage of the RE-400 is pretty impressive. Neither my belated RE-0 nor my HE-500 have as wide a soundstage or instrument separation. Granted, planar headphones like the HE-500 aren't exactly known for their soundstage, but my point stands.
As far as the bass goes, the RE-400 have much, much better bass in terms of both quality and quantity than the RE-0 were ever able to accomplish. It extends much deeper and has substantially more power (though this was a known weakness of the RE-0). This isn't to say that they are excessively bassy by any means, but they posses a surprisingly accurate and deep low end for an IEM.
The mids are also more present on the RE-400 than RE-0, which could be both a good and a bad thing. Since they have a pretty substantial bump at 2KHz, some might find this slightly unpleasant (some might describe it as "shouty" or "honky" mids). Headphones that people generally agree to have a "pleasing" and "laid-back" sound signature (like the HE-500) often attenuate the 2k frequency to mitigate this issue. However, in general the mids (like the rest of the headphone) are pretty darn accurate and to my ears sound better than the RE-0.
The treble is the only category where I would say the RE-0 have the upper hand over the RE-400. Since the RE-400 has a darker [deeper, richer, pick your semantically vacuous adjective] sound signature, the treble is slightly less present, which is most noticeable when listening to vocal music. This can also be determined through visual inspection of the measured frequency response, which shows a big dip in at 4KHz (the "in your face, presence" frequencies), and a slightly smaller dip at 8KHz ("sparkle").
To mitigate some of the issues I mentioned regarding the shortcomings I perceived with the RE-400, I made a sample equalization profile I'm using with MediaMonkey. If you don't believe in equalization or don't hear the same things I do, feel free to totally ignore it.
All in all, the RE-400 turned out to be an astonishingly good buy, and I think they categorically outclass their predecessor (the RE-0) in nearly every way and make few compromises with regards to sound quality, despite their price. I can't imagine getting much better sound for only $100.
