Just received unit 1 today and had a chance to play around with it for about an hour and a half. Here are my initial impressions.
Build quality is excellent, very solid and durable feeling, with a touch of luxury. Earpads are soft and comfortable, and I was easily able to keep it on for an hour without any complaints. However, they don't fully go around my ears so they fit kind of like hybrid on-ear/around-ears. Also, the pad covering the drivers touches my ears. I have big ears and a big head though. But if I had designed this headphone, I would have made the earpads slightly taller, wider, and deeper.
Plugging it into my iPhone 6 and listening to lossy files from my collection as well as lossless music from the Tidal app, I enjoyed it from the get-go. I found the sound to be likable, laid-back, and neutral to slightly warm in tone, with a polite top end, maybe slightly recessed midrange (vocals), slightly warm upper low end, and just right low end (not too boosted and not too anemic with nice extension).
After a few songs to get used to it, I then plugged my Senn HD600 into my wife's identical iPhone 6 and tried to level match the two as closely as I could by ear (of course the PM3 is a lot more sensitive than the HD600). Then I switched back and forth between the two on identical material. This gave me a lot more insight into the sound. The PM3 definitely has a slightly thicker tone, especially in the upper and lower bass. However, much of this might be due to the PM3 being closed and the HD600 being open. But strangely, even though the PM3 has more bass and is slightly warmer overall, the HD600 sounded more substantial to me, like there were real voices and real instruments in the room, whereas the PM3 seemed a little more artificial sounding and almost tinny by comparison. Maybe it's because the HD600 midrange is stronger and more realistic. I don't know if the HD600 has an unnatural boost in the midrange while the PM3 is more neutral, or if the HD600 has a more natural midrange and the PM3 has an unnaturally shelved midrange. However, I know that the HD600 midrange sounds better to me, making voices sound stronger and realer, and making percussive snaps more authentic. And the open HD600 has wider, larger imaging, which adds to the musical realism. Also, the treble on the HD600 sounds just a touch more refined and airy than the PM3's more subdued and polite treble, although I bet the HD600 is a little peakier on the upper range.
So based on an hour's worth of listening, I prefer my old HD600 over the PM3, even though the PM3 has deeper and tighter bass and I appreciate the sound isolation. I don't have any closed headphones around to compare the PM3 to unfortunately. The only closed headphones I've owned in the past are the AKG K553, K271 MKii, and Senn HD280, and based on my fallible memory, the PM3 is much better than any of those, being more neutral and open sounding and more refined. So, if I wanted to get a closed headphone to complement my open HD600, I would definitely consider the PM3, although I would need to compare it with some of the competitors in the marketplace.
I haven't tested it with my Oppo HA-1 amp yet, though. However, I would guess that the amp would cause the HD600 to sound even better and not make as much of a difference with the PM3, creating an even bigger gap between them in my perception, but this is only a guess. If the results turn out differently, I may post a follow up.
Keep in mind that these are just one person's opinions and I'm not a headphone hobbyist at all. Once I find a piece of equipment that I love, I tend to just stick with that one thing and keep it for like 10 years or more, and not clutter up my situation with multiple headphones for the sake of variety. So maybe I'm just really used to my HD600 sound. YMMV.
