Performance The PSB M4U 1 is a remarkably clear sounding headphone. The clarity isn't the result of boosted treble or midrange frequencies; no, the M4U 1 just sounds neutral (it isn't the sort of headphone that emphasizes bass). Switch over to the Sol Republic Master Tracks headphone and you'll definitely see it has a thicker, fuller sound balance, but the midrange and treble are muffled by comparison and the instruments sound farther away.
Sony's MDR-1R is a more accurate headphone than the Master Tracks, but it's still not as clear as the M4U 1. On the Drive-By Truckers' "Where's Eddie?" the M4U 1 brings Shonna Tucker's voice up close, and the band's accompaniment is set further back in the soundstage. The MDR-1R diffuses the sound, and flattens the recording's dimensionality.
I like the M4U 1's clarity, but with a brash recording like Prince's "Gett Off" the MDR-1R's softer sound takes the edge off. If you listen to a lot of low bit rate MP3s or streaming audio, the M4U 1's clearer presentation might not be desirable. That said, with better recordings you'll appreciate the music's sound more with the M4U 1.
The $350 Sennheiser Momentum is another highly regarded full-size headphone, and it was also a little thicker sounding than the M4U 1, but they're very close overall. I'd be happy with either one.
All these comparisons were done with an iPod Classic playing Apple Lossless tracks. At home, I tried the headphones with Schitt Audio Magni headphone amplifier to mix (it did improve the listening experience). Note: Using the headphone amp, the M4U 1 was still ahead of the MDR-1R in terms of clarity.
Back on the iPod, I compared the M4U 1 with the M4U 2 noise-canceling headphones, with the noise-canceling turned off. I was expecting the two headphones to sound exactly the same, but the M4U 1 came away the winner, sounding slightly more transparent and clear (the M4U 2 was slightly more distant sounding). That said, if you listen mostly in noisy environments, the M4U 1's small advantage would be masked by the ambient noise, so under those circumstance it would be better to use the active-noise canceling M4U 2.
Conclusion The M4U 1 is first and foremost an audiophile headphone. It was designed to be as neutral sounding as possible, so bass lovers might not be satisfied with its low-end prowess. It's also fairly comfortable to wear for hours at a time, but certain competitors, such as the Sony MDR-1R, are slightly more comfortable.
If you're trying to decide between this and the active-noise canceling M4U 2, it really comes down to where you listen to your music and if it's important for you to have noise-cancelling headphones to better muffle ambient noise. However, most active noise-canceling headphones don't sound as good as their passive counterparts, and that's the case here, too. The M4U 1 sounds slightly better than the 2, and at around $300, audiophiles who prioritize sound quality over everything else, it's hard to beat for the price.