Headphone aficionados often argue that open-back headphone designs often enjoy a small but worthwhile edge in terms of openness and transparency—a stereotype with which I might generally agree, but the M4U 2 seems for the most part to be an exception to the rule. It has none of the compressed, closed-in sound that some listeners associate with closed-back designs; on the contrary, it sounds unfettered and alive.
Are the M4U 2s fully the equal of today’s best roughly $400 passive headphones? I would say that, when push comes to shove, they do not, though the gap is amazingly narrow. Three great passive headphones that I think offer slightly higher performance than the M4U 2 would be the $395 Fischer Audio FA-002W High Edition (click to read the review), the $399 HiFiMAN HE-400 with Rev2 drivers (click to read the review), and the 399.99 (MAP) Shure SRH1440 (watch for Steven Stone’s upcoming Playback review). When backed by appropriately high-quality headphone amplifiers, each of these ‘phones can potentially offer a bit more midrange and treble detail, even more open and extended highs, and difficult to describe quality of heightened resolution and focus. But the tradeoff, please bear in mind, is that those competing ‘phones really need good outboard amps to work their full magic, whereas the M4U 2, which comes very close in overall performance, comes with its own amp already built-in and ready to travel. Add in the fact that the M4U 2 is a noise-canceller with headset capabilities, and you can easily see why the PSB headphone would likely be the go-to choice for pragmatic, real-world listeners (especially for those who find the idea of springing for an outboard headphone amp a little far-fetched in the first place).
Let me offer several more observations prospective M4U 2 customers might want to know about. First, unlike the overwhelming majority of noise-cancelling headphones I’ve tried, the M4U 2 offers a really sensitive and good-sounding passive mode that presents an easy-to drive 32 Ohm load to iPods and the like. Thus, should the batteries die and you don’t have a spare set handy, the passive mode is really very good and eminently listenable. Second, the M4U 2’s best sounding mode is without a doubt the Active Mode with noise-cancelling features turned off. Purists will find the M4U2 sounds, well, purer, more vibrant, detailed, and alive with the amp on than with it off. Interestingly, when the amp is turned off, the input impedance of the M4U 2 becomes a super easy-to-drive 10kOhms. The Active Noise Cancelling Mode sounds similar, but not identical, to the Active Mode, with bass and lower mids that are not quite a free flowing and expressive as when the noise-cancellation circuitry is disabled. According to published PSB specifications, distortion is somewhat higher with the Noise Cancellation turned on.
Noise reduction capabilities for the M4U 2 are very good, with no additive electronic noise from the noise-cancellation circuitry. Some noise-cancellers create a sonic “robbing Peter to pay Paul” scenario, where noise goes down in one part of the audio spectrum but seems to increase in other parts of the spectrum—a problem the M4U 2 simply does not have. I would say, though, that the noise reduction capabilities of the M4U 2, good though they may be, are not quite the equal of the unusually versatile and effective noise reduction circuits found in Audio-Technica’s superb new ATH-ANC9 noise-cancelling headphone. The tradeoff, though, is that the M4U 2 is unquestionably the more refined musical performer of the two, which we count as an important advantage in the PSB’s favor.
Caveats: Most of my reactions to the sound of the M4U 2s were very positive, though I do have a few minor nits to pick, most of which relate back to the fact that when powered up the M4U 2s have a lot of gain and are extremely sensitive. For the most part high gain and high sensitive are good things, but the one drawback is that any unintentional noise or hard, sharp transient sounds will be vigorously amplified right along with the music.
Thus, I found that when I first pressed the Stereo Monitor buttons on the M4U 2’s signal cables (with the power turned on), I heard a hard, intense “CLAAaack!” though the headphones. As near as I could tell these sharp transient noises didn’t do any real damage, but they scared the bejibbers out of me when they first occurred. Similarly, swiveling the M4U 2’s signal plugs within the output jacks of an iPod or iPhone occasionally induced surprisingly loud “ticks” or “pops” (possibly due to unseen dirt or grit in the jacks). I also discovered that, through the M4U 2, marginally noisy source components could suddenly start to sound annoyingly noisy. Any hum or sonic “hash” that’s present in the source will be reproduced through these ‘phones, loud and clear.