My biggest criticism of the MM 450-X, is that the aforementioned treble boost is exacerbated with the noise-canceling circuitry enabled. In A/B testing, I heard a combination of more treble and what sounded like an increase in overall gain with NoiseGard on. I'm assuming this treble bias is meant to counteract high-frequency noise that might make its way past the less-than-perfect earpad seal, and the result isn't unpleasant, but it does color the music. (In brief use watching videos, the treble bias wasn't as noticeable.) Still, given the inherent characteristics of noise cancelation, the MM 450-X is one of the better performing noise-cancelation headphones I've used.
The MM 450-X is equipped with an array of microphones on the exterior and interior of the earpieces. Many of these provide audio for the noise-canceling circuitry to work with, but they're also used for phone calls, Siri, and other voice-related functions. I came away disappointed with the performance of the MM 450-X during calls and with Siri. Voice quality during calls was only average, and my success rate with Siri was lower than with nearly every other headset I've tested.
Is the MM 450-X the Ferrari of noise-canceling headphones? It's certainly constructed well enough to outlast a couple generations of iPhone or other smartphone, and the Bluetooth implementation is deep, providing a huge number of control commands while maintaining simple access to the most common ones. I also found the overall audio quality, both wired and over Bluetooth, to be very good, with accurate soundstaging, crisp reproduction of instruments and vocals, and tight bass.
Where the MM 450-X falls short is in its performance as a headset and in its audio performance with noise-canceling circuitry enabled. While one could argue that you can't expect stellar audio from any noise-canceling headphone, the Sennheiser brand and the MM 450-X's price set certain performance expectations. What's disappointing here is that, to use my car analogy, while Ferrari doesn't make its own navigation system, Sennheiser has long made noise-cancelation systems. Though the MM 450-X is good, it's difficult to give an unqualified recommendation for a $450 noise-canceling, Bluetooth headset that doesn't have best-in-class noise canceling performance and functions poorly as a Bluetooth headset.
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