I posted a very brief comparison between the Pro 500s and the M-100s in the "Beats sues Yamaha" thread and Craigster asked me to state my impressions here.
To start, I have to say that my time with the Pro 500s was maybe 3-4 minutes and in that time I listened to the first parts of 3 songs that I listen to often.
Genie (Tell Me Your Wish) by Girls' Generation
Lucifer by Shinee
Rolling in the Deep by Adele
I like these three tracks because they each allow me to experience sound stage, stereo separation, bass and mid-range presence within a short time.
In the song, "Genie," the opening has the sounds of women chatting and walking in high heels on the left channel as they approach the listener before activating a car alarm on the right channel. It's my go to song to see how well the sound stage plays out on closed headphones which can sound congested (HD25-1 II, Portapro) or open headphones which sound insanely wide (SR60i, DT 990, HD 650) compared to my strictly V-MODA roster of headphones. I own the M-80 and the M-100, which have both received "better than average for a closed headphone" sound stage praise. The Pro 500s imaging of this sequence has a very similar spatial positioning to M-100s. The cute Korean chatter amidst the clacking of women's heels against the concrete sounds a fair distance off, although not quite on par with the open headphones I mentioned earlier.
"Lucifer" is fast-paced, boy band Kpop at its best with lots of fun bass, crazy stereo vocal flourishes, and a persistent hi-hat cymbal in the chorus positioned in the distant left channel. From what I remember, it sounded great on the Pro 500s and the only discernible differences being the mids are more in-your-face than on the M-100s and the bass is boomier and more impactful. Not boomy. Just boomier. The bass, to my ears, sounds more similar to the M-80s bass rendering than it does the M-100s, but it extends deeper that the M-80s.
As for "Rolling in the Deep," Adele has a great voice and I always felt the mastering of this song puts her too far back from the instrumentation. Do you guys get that as well? Every time I listen to this song, I always feel like I should scoot closer to Adele to fully experience her pain and try to console her, but it's like she's holed up at the end of a tunnel and as I scoot closer, she scoots away. It doesn't matter which headphone I use. She's always just out of reach. This feeling is much more apparent with the M-100s mid-range than it is on the M-80s. I think the Pro 500s fall in between these two, but closer to the M-80 in its forwardness.
Regarding the highs, on both the M-100s and Pro 500s I noticed no sibilance, a slight roll-off at the top, and a lack of really crispy, sparkly highs ala Grado or Sennheiser. The highs sounded very similar though the Pro 500s might have some more presence there as well. I can't say for sure.
A quick summary goes thus:
The Pro 500 sounds like an M-100 with similar mid-range and bass impact of the M-80.
Keep in mind this is only a 3-4 minute assessment of a headphone that I am comparing to 2 headphones which share the same manufacturer and which I have spent significant amounts of time tuning my brain to their sounds. 1 year for the M-80 and 4 months for the M-100 versus 3-4 minutes with the Pro 500. Take some salt with this tequila.