The PSB M4U 2 model's "active" noise-canceling powers are above average, and it's also the best-sounding pair of headphones of its type. The bass, midrange, and treble balance is flawless, so it sounds great with rock, electronica, jazz, classical, and in fact every type of music.
On the other hand, the Bose QuietComfort 15 headphones are superior noise blockers, but not so great with fidelity. Unlike most noise-canceling headphones, the M4U 2s sound their best in quiet settings, where you can turn off the noise-canceling circuitry and play music without draining the headphones' two AAA batteries. Also, using the headphones' internal amps without engaging noise cancellation boosts the volume a bit, so you can turn it down and play longer before the headphones' battery runs out of juice. You can't do that with the QuietComfort 15s.
On the plus side, with the M4U 2s I'm less aware of the mild noise-canceling "pressure" that's a common problem with many powered noise-canceling headphones. It's usually not so different from the pressure you feel on your eardrums when you're in a plane that's changing altitude.
In my noise-eliminating tests, the M4U 2 headphones handled air conditioner whooshes, traffic roars, and bus thumps and rumbles with ease. In fact, the louder the noise, the more impressive the M4U 2s sound. True, they won't completely hush the noise (no noise-canceling headphones can), but the M4U 2s definitely take the racket down to a more tolerable level.
The Audio Technica ATH ANC-9 headphones are even more efficient at canceling noise, but they don't sound as good as the M4U 2s during music playback -- it's not even close.
I do most of my sound quality evaluations on noise-canceling headphones in noisy environments, and the M4U 2s performed beautifully; during tests, I kept noticing their beautiful midrange.
I'm doing the bulk of my M4U 2 listening tests with my iPod Classic, but the M4U 2s are also among the very few noise-cancelling headphones that sound even better in a quiet room with the noise cancellation processing turned off. I paired the headphones with myheadphone amp with great results, but I'm sure you'll also get a similar experience with an AV receiver. Without a doubt, the M4U 2 headphones are the best-sounding dual-purpose (indoor/outdoor) noise-canceling headphones I have tested.
Priced at $399, PSB Speakers' M4U 2 Active Noise Cancelling Headphones may be one of the most expensive pairs of noise-canceling headphones on the market, but they're also one of the few that can still play after the batteries are drained. If you're an audiophile in the market for noise-canceling headphones, but you don't want to sacrifice sound quality, the M4U 2s are the pair to buy.