Can't decide if you want a wired or wireless pair of headphones, or one with active noise cancellation or without? If you've got the cash, you can have it all with the Sennheiser MM 550-X ($499.95 direct), an extremely well designed set that covers practically every situation you'd ever encounter. It also sound pretty amazing. It's not the best $500 pair of headphones we've heard—wired models still win in the end—but it certainly sounds fantastic, even over Bluetooth. Sennheiser's active noise cancelling is somewhat less successful, as it's not the most effective we've heard and also affects the otherwise pristine sound quality more than it should. But even so, this headphone pair is still an easy recommendation.

Design, Accessories, and Controls
The MM 550-X SEE IT is a closed, around-the-ear design, and weighs 6.3 ounces with the battery inserted. Both the earcups and the headband are heavily padded, and the soft padding feels quite comfortable. You get quite a lot of passive noise cancelling just from the tight seal the earcups form with the MM 550-X on your head. The headset is also a bit lighter than some larger pairs of wired headphones, like the venerable Sennheiser HD 650 ($499.95, 5 stars) and the Bose QuietComfort 15 ($299.95, 4 stars). Even so, it feels quite rugged; the plastic, folding hinges in the headband and around the earcups in particular look especially sturdy. 

Sennheiser packs in a number of accessories. In addition to the headphones themselves and the rechargeable battery, you get a USB charging cable, a power plug with adapters for different countries, a stereo audio cable (2.5mm to 3.5mm) for use in wired mode, a 1/4-inch headphone adapter, an in-flight adapter for use with double mono jacks on airplanes, and a rugged, zippered cloth carrying case. The manual comes as a PDF file burned onto an included CD-ROM.

The controls are extremely easy to use even with the headphones on your ear. The center of the oversize five-way control-pad on the right earcup is large enough for an entire fingertip, and features a raised plastic circle around it, so that you can't miss it. Top and bottom control the volume, while left and right skip to the next track in either direction, all on devices that support remote control over Bluetooth. You can also hold down the Skip buttons to fast-forward or rewind the track. The bottom of the right earcup houses Bluetooth, NoiseGuard, and SRS buttons (more on these in a moment), as well as a 2.5mm jack for using the headphones in a wired configuration. The left earcup only has a covered micro USB charger port.

Hold down the multifunction button for five seconds, and the MM 550-X goes into pairing mode. You'll never miss that; not only does the control pad pulse red and blue, but the SRS and Bluetooth buttons light up as well. I had no problem pairing the MM 550-X with an Apple iPhone 4. The Multifunction button also handles accepting, rejecting, and ending calls, and you can transfer the ongoing call between the phone and the headphones—or activate voice dialing by pressing and holding the Bluetooth button.

Sound Quality and Noise Cancellation

Sennheiser's big pitch with the MM 550-X is apt-X audio encoding, which the company claims delivers wireless audio quality that is indistinguishable from the highest quality wired connection. Here's the trick, though: It only works if the sound source also supports apt-X. My iPhone 4 doesn't, but even so, I heard pristine audio in both wired and wireless modes. Both modes sounded virtually identical. I heard the tiniest bit of compression artifacts in wireless mode, in a slightly more closed soundstage and a touch of occasional harshness, but I had to strain to hear it.

Aside from that, sound quality was excellent overall. In The Knife's "Silent Shout," our standard bass test track, I heard a touch of distortion at top volumes, but none at normal listening levels—or even beyond. The overall sonic signature was warm, with a full-bodied low-end, exceptional low-end extension, a smooth midrange, and airy highs that didn't stick out but were still full of detail. On Queen of the Stone Age's "The Lost Art of Keeping a Secret," it was easy to hear the multiple guitars and other instruments across the soundstage, while the rhythm section still had plenty of punch.

Pressing and holding the NoiseGuard button for two seconds activates the MM 550-X's active noise compensation system. It does this via a pair of tiny mics near the earcups that focus on ambient noise in the 20Hz to 1KHz range, and then use this noise as a template for generating an opposite wave, 180 degrees out of phase, to cancel the noise out. Press and hold a second time, and you'll turn NoiseGuard off.

With NoiseGuard on, the MM 550-X did a nice job of muting some strong office fans of various sizes. But you could still hear them more than you could with, say, the Bose QuietComfort 15 SEE IT. My main issue with NoiseGuard, though, was the reduction in fidelity; it turned the MM 550-X into an average pair of headphones you could buy for about $50, in terms of sound quality. The warmth disappeared, I heard the typical artifacts in the midrange that often betray active noise compensation, and nearly all of the MM 550-X's marvelous bass extension vaporized. Bose QuietComfort models are by no means musically neutral, but they seem to do a better job of compensating for the active noise cancellation circuitry.

TalkThrough, Other Features, and Conclusions
The TalkThrough function takes the audio recorded from the tiny mics and actually pumps it in the earcups. This way, you can hear what's going on around you without removing the headset, while still filtering out more distant background noise. You activate this by just briefly pressing the NoiseGuard button, instead of holding it down. When you do this, it pauses the music so you can talk to someone in the room; turn off TalkThrough, and the music will begin playing again.

Meanwhile, the SRS button is, well, as the SRS button. With SRS WOW HD engaged, the highs are exaggerated in the 5KHz region, as if you're listening to a cassette on a car stereo, with little top-end air or detail around them. The button also amplifies the bass even further, but with extra mud, as if the "bass" control on a cheap mini system is turned up without much in the way of granularity. My recommendation: Leave it off.

In my tests, MM 550-X wasn't that great at handling voice calls. I heard the other caller, but their voice sounded computerized, and I could barely hear my own words because of the tight seal of the earcups. I was hoping TalkThrough would be a good workaround for this problem, but if you activate it during a voice call, it does the same thing as it does with the music—it mutes the audio source and turns on the external mics, so you can't hear the caller anymore.

According to Sennheiser, the battery should provide 8 hours of continuous listening with Bluetooth and NoiseGuard (active noise cancellation) enabled, 10 hours over Bluetooth without NoiseGuard, and 20 hours in wired mode with NoiseGuard activated. It will last forever in wired mode without noise cancellation, because then it works just like a regular pair of headphones. The LED indicator flashes red when the battery is almost dead.

One other minor quirk: You need to take care when disconnecting the micro USB charging cable. Otherwise, you'll pull the battery clear out of the left ear cup, which isn't good if you don't notice it, and then leave the house without the battery inside. But there's a reason for this design: You can charge the battery pack either when it's inside the earcup or out, since the battery itself contains the micro USB charger port.

Overall, the Sennheiser MM 550-X is an extremely flexible set of headphones. You're paying for all of this technology, of course, and not all of the money is going into the actual sound quality. The MM 550-X sound good enough that you'll feel like it's money well spent, especially when using it wirelessly. Its active noise cancellation, while not class leading, is still fairly effective; if you want the best and don't need a wireless pair, the AKG K 490 NC ($249.95, 4.5 stars) or the older Bose QuietComfort 15 will still perform better there. If you don't specifically need Bluetooth compatibility or active noise cancellation, the Sennheiser RS 220 ($599.95, 4.5 stars) remains our Editors' Choice for high-end wireless headphones, with its stellar sound and comfort, although it is really designed for home use. The Sennheiser MM 100 ($149.95, 3.5 stars) is a more affordable and practical wireless option that still delivers solid performance. And if you don't mind an in-ear design, the wireless Sony XBA-BT75 ($299.99, 3.5 stars) sounds good and is very easy to tote, although it's still a little big when compared with standard stereo earphones.

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