The MM 550 is well built, with comfortable padding and metal hinges that allow the large earcups to fold up for travel.
Controls on the right earcup handle volume, Bluetooth, SRS WOW HD sound enhancement, NoiseGard 2.0 active cancellation, and iPod playback.
Without SRS or NoiseGard activated, the MM 550 offers solid detail and a low end that favors tightness and definition over depth and power. Sadly, when we enabled noise cancellation, the bass nearly disappeared and the midrange became irritatingly harsh. Adding SRS to the mix (or using it by itself) only hollowed out the music in the midrange and added some muddy upper bass.
Call quality via the built-in mic is so-so, though this may vary with different phones. The active noise cancellation is reasonably effective, but it's not as tomb-like as the Bose model. Without power, the MM 550 still works as simple wired headphones.
Overall, the MM 550 makes a good-sounding--if very expensive--set of headphones, and its versatility alone will be worth the steep price for many.