Though it's not necessarily designed for exercise (nor is it actually water- or sweat-resistant), the Audio-Technica AH-CKX9iS SonicFuel offers a secure enough fit that many users will feel confident wearing them at the gym. The true strength of these $99.95 earphones, however, is the powerful bass response they mix with a pleasant brightness. The high-mids and highs are well-represented, making sure the substantial low frequency presence doesn't muddy things. Even better, the AH-CKX9iS doesn't distort on deep bass tracks, making the pair an affordable, high-performing option that will appeal to lovers of tastefully boosted bass and solid overall balance.

Design
The AH-CKX9iS is available in black-and-gray with a black-and-gray cord, or in white-and-gray with a black-and-blue cord. Its earpieces are on the chunky side, partly because the silicone cover for each is a bit bulky, but this isn't a bad thing. The covers aid immensely in stabilizing the overall fit of the earphones, using a protruding fin we've seen before on Bose models like the SIE2i. A flat, linguini-esque cable descends from each earpiece, meeting around mid-torso at the in-line remote and mic.

The remote is a single button coupled with a volume slider. The slider is a little awkward to use but it can at least make volume adjustments when most one-button remotes can't. Still, some users will wish for a three-button remote for easier track navigation and playback control.

The AH-CKX9iS ships with four pairs of silicone ear tips and one pair of Comply foam ear tips. If you want an extra dose of quiet and a more secure fit, the Comply tips are the superior option. They also help add a little extra bass response, because they efficiently seal off the ear canal. The silicone covers for each earpiece are also removable and come in three sizes, all of which are included along with a hard-shell plastic carrying case for keeping all of those parts together.

Performance
On tracks with serious sub-bass content, like The Knife's "Silent Shout," the AH-CKX9iS delivers laudable, thunderous bass response that manages to blend well with the mix rather than overwhelm it. The high-mids and highs are well-represented on this track, too, and the earphones manage to avoid distortion even at top (and unsafe) listening levels.

Bill Callahan's baritone vocals on "Drover" receive a healthy extra dose of low-end richness (which they hardly need), as do the drums. In both cases, however, the bass presence is complimentary; it doesn't make the mix muddy or Callahan's voice overly resonant. Purists will wish the bass dialed back a bit more, but it suits most modern mixes quite well, thanks to the presence the AH-CKX9iS has in the high-mids and highs. It lets vocals remain somewhat crisp and guitar strums retain their brightness, even when paired with substantial low-end.

On Jay-Z and Kanye West's "No Church in the Wild," the kick drum loop receives a decent amount of high-mid presence that gives its attack some edge, but it's not as potent and cutting as it is on pairs that are a bit more boosted in the high-mids. The sub-bass synth hits that punctuate the beat have a deep presence that isn't over-the-top, but still commanding enough to steal some of the spotlight when they hit. The vocals manage to float over this dense mix without being overpowered.

Classical tracks, like John Adams' "The Chairman Dances," benefit from the added bass presence since the bass isn't particularly overwhelming to begin with and lacks the gobs of sub-bass many modern pop and hip-hop mixes have. The boosting is subtle, adding some roundness to the lower register strings without shifting the focus from the higher register strings, brass, and percussion.

Purists seeking flat response will likely find the AH-CKX9iS a little too bass-heavy, but it should please anyone who likes a little extra low-end in the mix without overpowering it. If you want a slightly more balanced response in this price range, the Editors' Choice TDK EB950( at Amazon) is a solid option. For a more intense, booming bass experience, the SOL Republic Amps HD In-Ear Headphones( at Amazon) should satisfy. If you like the idea of the AH-CKXi9s's balance-with-a-side-of-bass, but are looking to spend far less, consider the half-priced RHA MA450i. For $100, however, the Audio-Technica AH-CKX9iS SonicFuel is an excellent-fitting earphone pair offering a high level of distortion-free performance.

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