Like its fit, the K551's sound is occasionally frustrating, but ultimately quite rewarding. Sonically, the K551 bears a strong resemblance to the K701, and that's mostly for the best--the K551 has the least "closed" sound I've heard from a sealed design. Like the K701, the K551 delivers a truly impressive amount of instrumental detail, and it sounds crystal clear, particularly in the midrange and treble frequencies. This detail and clarity are due in part to a somewhat bright balance, thanks to lower frequencies that are precise and clear, but not as visceral or powerful as with some other headphones in this class (a common complaint about the older K701, as well).
Bass aside, there's another caveat to consider: The K551's short cable and inline remote imply use with a portable device, but the K551's brightness exacerbates the faults of portable players, with their grainy audio output and low power. As a result, with a poor source, you'll hear a bit of high-frequency harshness in some recordings. To get your money's worth, you need to give the K551 some special treatment. Get a good fit, and use the K551 with a headphone amplifier, a nice digital-to-analog converter (DAC), and some lossless-encoded files, and it'll sing.
All this is to say that in the right setting, the K551 will amaze you by revealing musical nuance and detail that most other headphones—including the other models here—fail to capture: It will let you hear new and wonderful things in your music. If your primary purpose is portable listening, however, be aware that those models are a better match for portable sources—and the M-100 and Momentum are easier to transport, too.
Speaking of world-class headphones, Sennheiser has a few of its own; though like the K551, most of them aren't portability-oriented. The $350 Momentum (5 of 5 rating) aims to change that. Like Skullcandy's Aviator, the Momentum looks as though it would nicely complement a bomber jacket and aviator sunglasses. But unlike the Aviator, the Momentum also has a classiness almost on a par with the "scotch and cigars" executive look of Bowers & Wilkins's P5 Mobile Hi-Fi Headphones (4.5 of 5 rating).
The Momentum's look starts with a leather-wrapped, stainless-steel headband, which, like those on the K551 and M-100, could use additional padding. At either end, a central channel in the headband provides an attachment point for the earpiece. The earpieces slide (remarkably smoothly) along that channel to adjust the fit. Like those of the M-100, the Momentum's earpieces are small enough that they're effectively supra-aural on users with larger ears. The earpieces are a matte plastic that matches the color of the leather used for the headband and earpads. The color choices are brown (my favorite) or black; the latter adds red trim that gives the Momentum a little of the M-100's edginess.
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