If you're looking for flat response-style headphones that color the sound very little and offer an accurate frequency response, let me save you some time: No need to read this review. The JBL Synchros S400BT is aimed at those of you who live for massive bass, and these $229.95 headphones will not disappoint in terms of low-frequency response or power—there's no distortion at top volumes, either. The highs are very sculpted as well, so the Synchros S400BT rarely sounds muddy—this is a crisp sound with seemingly endless low-end. The control panel is touch sensitive, and easy to accidentally tap and pause music or worse (more on that later), but otherwise, with its included audio cable for wired listening, the S400BT is an excellent Bluetooth option for bass lovers.

Design

Offered in a black or white design, the Synchros S400BT features a stainless steel headband with ample cushioning on the underside. There are some metallic design accents on the earcups, and the earpads are also quite plush, making for a very comfortable fit. The circular pads are on-ear, but large enough that they feel like a circumaural (around-the-ear) pair that presses against the jawline a bit, as well. The headband, like other recent JBL models, features numbered click-stops for a precise, balanced ear-to-ear fit, and it also folds down at hinges above the ear into a more compact shape.

On the left ear, there's a touch-sensitive control panel (discussed below), a jack for connecting the audio cable for wired listening or the charging cable, a Power button, and the Bluetooth pairing button.

Pairing the Synchros S400BT with an Apple iPhone 5s was a breeze, and the headphones will auto-pair when you power them up next. (You can also pair via NFC if you have an NFC-enabled device.) However, operation of the headphones will require reading the manual and paying attention. For instance, if you pick the headphones up while they're paired and absent-mindedly hold your thumb on the outside, touch-sensitive panel of the left ear, you'll be in for a surprise! Touching this panel and keeping your finger there creates a pulsation on the outside rim of the headphones. While you wonder, "Why is it doing that?," the headphones command your phone to dial the last person you spoke to.

It's also quite easy to let a single tap or swipe intended to play a track or adjust the volume linger just slightly too long and achieve the same result: You will call the last person you spoke to, regardless of whether you called them or they called you.

Interestingly, the Quick Start Guide doesn't mention this extra feature, though it outlines other basic features of the touch-sensitive panel. Swipes forward and backward control track navigation, up and down control volume (the volume control is independent of your music source's volume), and single taps answer or end calls—or if you're not on a call, play or pause your music. Just as it's easy to accidentally dial someone, it's also easy to accidentally play or pause your music when adjusting the fit of the headphones, so you have to learn how to do so without touching the control panel.

Included with the Synchros S400BT: A USB charging cable and an audio cable for wired playback (both connect to the same jack on the left ear, however, so you can't charge and listen to wired audio simultaneously). JBL estimates the battery life on a full charge to be roughly 15 hours of music playback and 20 hours of talk time, though your volume levels will have a lot to do with your actual results. The headphones fold down into a handsome black rubberized zip-up case that's included as well.

Performance

On tracks with intense sub-bass content, like The Knife's "Silent Shout," it's hard to say it any other way: The Synchros S400BT sounds like a club PA system, delivering incredibly powerful bass as if a subwoofer were implanted in your skull. Maybe that's a slightly poetic exaggeration, but you get the idea—purists seeking flat response will be horrified, and lovers of booming, deep low-end will hardly be able to contain themselves. Even at top volumes on both the sound source and the headphones—a level you shouldn't really be listening at—there's no distortion, just thunder. Of course, to deliver low-end this big, you need to have some extra crispness and brightness in the high-mids and highs, or it will sound like a muddy mess, and the Synchros S400BT dutifully adds these higher frequencies boosts to the sound signature.

On Bill Callahan's "Drover," however, the semi-ridiculous bass presence starts to unravel a bit. Callahan's baritone vocals don't need extra help in the low frequency department, but here they get gobs of extra low-end richness. Again, the high-mids are also sculpted enough, amazingly, that his voice still sounds crisp and clear, despite its extra-large helping of bass. The drums, however, sound like a herd of buffalo stampeding—an exciting sound, without a doubt, but the drums on this track really don't need much bass boosting at all, and here they get tons. Does it sound accurate? No. But will it excite certain listeners who live for the sound of booming low-end? You know who you are, and yes, you will love it.

Jay-Z and Kanye West's "No Church in the Wild" sounds menacingly powerful through the Synchros S400BT, the thump of the kick drum loop armed with plenty of high-mid presence to slice through the mix while the low-end sustain steals the spotlight. When the sub-bass synth hits that punctuate the loop occur, they are intense to say the least. Somehow, the vocals manage to float over this mix, but thanks to the rumbling lows, they're not the main attraction.

Classical tracks, like the opening piece in John Adams' "The Gospel According to the Other Mary," sound, well, very unnatural. I'm still getting used to this new recording, but I've listened to it several times already, and through the S400BT, I heard lower register strings parts that I didn't really know were there. So in a certain sense, it's educational, but the idea of lower instruments blending with the rest of the philharmonic instead of overpowering it is a bit lost here. This is the sound of a punched-up, highly sculpted movie score, applied to meticulously recorded classical pieces. I got a kick out of it, to be honest, and I suspect it would make some people who think they're not into classical music more interested, but purists will obviously want to opt for headphones with less bass boosting.

If you love the idea of monstrous bass response, but find that the Synchros S400BT is a bit out of your price range, the SOL Republic Tracks Air is a slightly-less expensive option, while the Outdoor Technology DJ Slims is a solid, inexpensive Bluetooth pair. If it's a more realistic audio balance you seek from a Bluetooth pair, the Harman Kardon BT is a solid option with plenty of bass, but not as much as the Synchros, while the non-wireless AKG K545 is also worth checking out. For $230, the JBL Synchros S400BT is a boosted bass-lover's wireless dream come true, and its price and at times tricky, accident-prone control panel should not be considered deal breakers if you want some serious bass—and Bluetooth—in your headphones.