I'll go a little more in depth on my impressions now that I've had a pair on hand for the weekend. The setups that I've used at home are all visible in my signature below, and the combo I heard before taking it home to try with me was a Benchmark DAC1 > SPL Auditor, which used to be my work chain before I sold my PS1000 last year.
The PM-1 is significantly warmer than the HD800, T5p, and PS1000, but isn't as warm as an LCD-2, TH900, or W3000. As I've said, it occupies a spot in the TOTL realm that is very simliar to the spot that the HD650 has in the mid-fi world. I could also make a comparison to the place that the SR007MKII holds in the statement headphone realm, sitting between genre-dependent niche models like the ED10 and Muramasa and all-out detail seekers like the Abyss and SR009.
Around the $1K price range, there are still only a few models that are truly suitable to be driven from portable sources. Even the PS1000, a traditional dynamic with a 32-ohm impedance, is fairly lackluster when driven directly from an iPod, which is why I always kept a decent setup at work with them. The only other model at this price bracket I can think of that truly work to near its full potential from a DAP is the T5p, but Beyerdynamic's design is aging and uncomfortable compared to the PM-1, and doesn't really beat it at anything. The PM-1, on the other hand, is fully capable of giving 98% driven directly from a decent DAP, and that 98% sounds great.
Now that a general comparison of sound signature and portability have been established, all the rest of my impressions are derived from listening on setups 1 and 3 in my signature. The AMC 1100 isn't exactly spec matched well to the PM-1 via the rule of eight (with 100 ohm output impedance), but it still sounded top-notch to my ears. I didn't find that the HeadSave Classic improved my listening much on the iPod, but I paired it with the ODAC on occasion to see how the PM-1 would respond to a warmer amp. It's not a stellar portable amplifier, but it's all I had on hand. My thoughts are as follows:
As a lover of the HD650 for involved, relaxing listening sessions, I have to say that I believe the PM-1 is a notch above it in every sonic category, and does everything the aging Senns do, but better. Now that the industry has moved up in price and in development investment over the last ten years, I believe that the PM-1 is a worthy successor to the HD650 as a top-rung headphone for those who love for their music to speak for itself, and not have to listen to the harshness or peaks and valleys introduced by TOTL equipment. It's effortless, euphonic, and involving, with an intimate soundstage and outstanding instrument placement emerging from a black background. If somebody had told me that Sennheiser had introduced these as an alternative to the HD800 for lovers of the classic Sennheiser flagship sound, I would not have batted an eye.
With all of this praise coming from my mouth, it's important to note that the PM-1 is not for everybody. As I've stated, I absolutely adore the HD650. Though I keep a variety of more neutral and resolving cans on hand for critical listening, most of the time my mood veers towards sitting back, closing my eyes, and enjoying the music as a whole instead of focusing on certain details. If you like the presentation of headphones like the KNS8400, M500, HD800, or SR009, these are going to be too warm for your liking. On the other hand, if you love the TMA-1, PRO900, or LCD-2, these are going to be too bass-light and perhaps less impactful than you would like overall. The PM-1 is best suited to somebody who wants a warm yet fairly balanced sound signature, or maybe an alternative to a more resolving or visceral can for casual listening.
TL;DR In my opinion, the PM-1 is essentially the spiritual successor to the HD650 with all the improvements brought about by a decade of progress in the industry.