The 400vis' sound signature is typical of Ultimate Ears's 'phones - with a focus on the high and low extremes of the frequency spectrum. The high-end is very detailed and manages to largely avoid fatiguing sibilance unlike the 4s, but without the warmth of a prominent mid-range to back it up, there's occasionally a hint of artificiality to the sound. Vocals can sound slightly thin compared to more capable, if more expensive, models.
Pumping-up bass levels to dazzle the ears is a common tactic of entry-level earphones, and while part of the 400vi's bass is oversized, it's not the low-end bloat we're used to. There's a bass swell, but its far down the register, giving bass guitars and kick drums an unusual level of prominence in the mix without dominating the whole sound. It doesn't boom like a pair of DJ headphones might, but it isn't perfectly proportioned with the rest of the sound. This effect was very noticeable straight out of the box, and lessened after 10 hours of listening or so, but never disappears completely.
The effect of this bass emphasis varies depending on what's being played. It can supply a rich tone to a lone acoustic guitar, add that satisfying subwoofer-style sub-bass to an electro track or let a kick drum and bass guitar steal the limelight from a female vocalist.
The sound is otherwise extremely detailed and restrained, with good separation and a wide stereo image. If this is a basshead's earphone, it's an unusually tasteful one. Spend more and you'll get a slightly more natural sound, a warmer sound or some more detail, but you will usually have to spend significantly more to step up to that next level.
Sound isolation is above average with the right rubber tip in place but as with any IEM, there's significant microphonic rumbling, usually caused by the cable knocking or rubbing against your clothing. This can be cut down by wearing the 400vis over your ears. Although they don't appear to have been designed with over-the-ear use in mind, we found it perfectly comfortable.
The "vi" of the Ultimate Ears 400vi's name means this package has a built-in microphone, which is part of the on-the-cable controls, for use as a phone headset. We tried it out with a couple of smartphones and had no trouble, even with an Android phone not officially supported according to Ultimate Ears's website. The call button worked perfectly and the audio was relayed through both earphones loud and clear.
We had less luck with the remote control functionality built into the cable. The mid-cable control housing has three buttons that are meant to act as play/pause, volume control and track change buttons with an iPod Classic, iPod Touch or iPhone. They worked just fine with our test iPod Touch and iPhone, but refused to work at all with a 160GB Classic model.
The Ultimate Ears 400vi earphones are an improvement on their predecessors, the Super.Fi 4s, with a sleeker design and less artificial sound. They provide plenty of very low-end bass and oodles of detail to match, but thanks to a slight over-emphasis of this low end, the otherwise accurate sound is thrown slightly off-balance.