Editor Ty Pendelbury and I put them up against the $300 Bowers & Wilkins P5, a headphone that we both like and Ty uses on a daily basis. The M500s have a little more bass and are a bit warmer (not as bright). But the P5s offer up more midrange detail. Ty gave the slight nod to the P5s for sound, but he liked the design and build quality of the M500s better (he finds the P5's design a bit ostentatious even though he wears them regularly).
"The KEF had a warmer sound and a more cohesive soundstage on 'I Am An Ape' by David Byrne and St. Vincent," Pendelbury remarked. "The bass synth throbbed with greater insistence on the KEFs. The extreme left/right percussion effects seemed more tightly married to the music on the KEFs, while the P5s seemingly threw them out as separate entities."
In short, detail hounds will prefer the P5s but if you want a more "musical" and less taxing presentation the KEFs will be more in your wheelhouse.
If you're wondering how these sound versus the $200 Bowers & Wilkins P3, the P3 is a bassier, duller-sounding headphone. Try the M500s after the P5s and you'll think the M500s sound a little duller. But try the M500s after the P3s and the M500s seem much brighter.
For kicks I also ran them up against the new Beats Studio, which retails for $300. It's a noise-canceling model, so the comparison isn't exactly apples to apples, but it's certainly a popular headphone in this price class, and Beats has improved the sound in the new model (it's better balanced). The Beats Studio served up more bass than the M500, but the M500 was more accurate; I preferred its sound, but if you're looking for noise-cancellation, the Beats certainly has its appeal (the new Studio sounds better than the Bose QuietComfort 15 but the Bose's muffling abilities are superior).
If you're wondering why I'm not being a bit more enthusiastic about M500s' sound, it's because they just don't have quite enough body (read: fullness) to really blow you away. For example, the $350 Sennheiser Momentum has richer sound (the M500 is the more comfortable headphone, however). And when I compared with the Audio-Technica ATH-M50s, one of our reference midrange over-the-ear models, the $160 Audio Technica came out the winner. But again, the KEFs are a more comfortable headphone and better designed for mobile use -- they also work well as a headset, though the buttons on the inline remote are a touch small.
What makes a good headphone isn't just good sound but a combination of good sound, comfort, and build quality. As far as the M500's sound goes, I didn't get that wow feeling that makes you want to go back and listen to your whole music collection again to hear how it sounds through these headphones. But the M500s still sound very good, delivering relatively accurate, well-balanced sound with good detail and strong bass. In all, that makes KEF M500s an excellent set of everyday headphones that works well with a variety of music -- whether you're using them at home or on the go with a mobile device.
At $300, they may not be a bargain, but neither are most $300 headphones, and at least these are built with a good amount of metal, not plastic, parts.