Ok, quick impression - please keep in mind this is my opinion (as is everything when it comes to sound), and that I have only spent two days with it. I'll be doing more listening and have my wife try them as well.
So my very first impression was... they sounded kinda boring. I know Tyll at InneFidelity said similarly in his review of the PM-3, and I was careful not to specifically listen for what he mentioned. But the simple fact is the PM-3's treble is very laid back - I would almost go as far as to say it sounds rather veiled. Everything sounds a bit muffled and I have trouble picking out the textures of instruments. This hurts the timbre of the instruments a lot - especially any kind of metallic instrument - to me the treble's just missing something that makes them sound natural or realistic.
Now, in all fairness, it is possible that I am used to the sound signature of the AKG K550/K553 (both of which I own), which are my daily headphones and have a significantly more forward treble; but I've heard live instruments before and to me, the PM-3 just doesn't reproduce the kind of sparkle and texture that should, realistically, be present. I suppose it is also possible that the demo units have been damaged by abusive users - I can't really tell if that's the case or not (aside from the treble issue, I don't hear any sort of crackling or artifacts that would indicate diaphragm damage).
The one positive thing I noted (and Tyll mentioned this as well) is that vocals - especially male vocals - have a very hefty weight to them on the PM-3. There seems to be an area in the upper bass that's boosted to achieve this effect, and I personally quite like it. Bass extension is decent - though the PM-3 has a stronger emphasis on mid-bass, whereas the K553 has an emphasis on sub-bass, so although the bass quantity is comparable on the two sets, the K553 seemingly reaches deeper.
The midrange is very neutral - neither forward nor recessed - and decently clear. But I can't help but feel the overly recessed treble hurts the presentation of the mids, which is otherwise excellent.
Quick Comparisons:
AKG K553 - These are surprisingly capable headphones and not at all what I expected when I paid $120 for them on MassDrop. I actually find them to be more resolving in comparison to the PM-3, likely due to the more forward treble. Unlike the sometimes bitey K550, the K553 seems to stop just on the edge of sibilance on harsh recordings, while retaining excellent clarity and extension. The K553 has significantly more air and wider soundstage compared to the PM-3 due to this, which sounds rather closed in. Both headphones have grain-free treble (which is not a surprise in the case of the PM-3, since they are magnetic-planar, but for the dynamic K553, it is quite an achievement), and comparable bass quantity (albeit with different emphasis, as aforementioned). When I first got the K553, I felt the bass sounded bloated and loose (since I had gotten used to the K550, which now in retrospect was actually a bit thin in the bass), but comparing it against the PM-3 I actually find the K553's bass to be just as well-behaved. The midrange on the PM-3 is thicker and more lush to my ears, which makes the K553's midrange sound a bit recessed in comparison - but on the other hand I find the K553 to be cleaner-sounding, with a darker background (a bump in the midrange would likely hurt this presentation)
Audio-Technica ATH-A900X - I bought these for my wife at the same time I bought the K550 for myself a few years back. They are tonally more similar to the PM-3 in my opinion, albeit with a sparklier (though grainy-sounding) treble. Bass extension isn't as good as the PM-3, though it has more of a mid-bass boost. The mid-range is similar, though the PM-3 sounds somewhat cleaner.
Will listen to these for a few more days and see if I change my mind about them. At the moment though, I can't say it's an "upgrade" from me... if I do end up getting a pair it would be more like a sidegrade.
