A sensational post and read, providing real insight and distinctions...........thoroughly enjoyed reading it.
Well, I wasn't suggesting the Piano Fortes didn't sound natural to me because they aren't neutral. They simply don't sound natural to me. Similarly, a perfectly flat FR doesn't really sound natural to me either. Really though "natural" is kind of a problematic term to employ, as it kind of has a very personal meaning I think.
To that end, I'm of two minds with regard to what you're saying.
On the one hand, I think it's a little problematic to assume the Piano Forte is conveying the way something is mastered "naturally." It may sound natural to you, but can we really presume that's the way the engineer intended it to sound? Ideally, a more neutral pair of headphones will allow the mastering to come through without adding anything to it. In other words, they reach that baseline and tweak it to sound natural to them using their own headphones and not the Piano Fortes. Seems obvious enough, but really it speaks to the fact that listening with very colored headphones like the Piano Fortes is more about what we personally conceive as natural and not the producer. And that's fine. We tweak things to suit ourselves. However I think there may be a tendency to conflate the window with the scenery outside, if that makes sense.
Assuming the producer tunes something to be natural to his / her ears, the Piano Forte's very distinct coloration is adding to that on our end, doing more than simply letting what the producer "intended" through the gates (unless he / she masters specifically with the PF in mind). I suppose I'm saying that, for me, the truth of the 1601/1602 doesn't lie in its allowing that truth to come through, but rather in its offering a very unique and singular vision of that truth. Here's another way of looking at it: if the Piano Forte's truth is really universal, then why is it so difficult for so many people to get? More people don't like the sound of these than do in my experience. Perhaps it's a bit naive on my part, but I tend to think that "natural" sounding music is more obvious and has more of an immediate, recognizable appeal. No, to me this is exotica.
Speaking of conflations, I think there's also a tendency to confuse "flat" with "analytical." I've seen it time and time again: people calling headphones like the K701 neutral or flat. That's simply not the case. Some people mistakenly associate headphones with big treble tilts and massive detail emphasis as being flat. In actuality, a flat headphone like the Paradox doesn't sound very artificial or analytical in my opinion.
On the other hand, for my personal tastes, I think audio playback is at its best when the entirety of the system is considered. This sort of holistic approach is common in higher-end Japanese audio like Kondo Audio Note and Shindo Labs. They seem to favor a more organic and unified view. In this sense, I absolutely agree that achieving a "natural" sound isn't just tuning something to be flat. There's a certain vitalism, a certain... spirit to music, and one needs to consider every component of the audio chain. It's not just the headphones that contribute to good sound, but the amp and source, and beyond that the engineering and mastering, and beyond that the microphones and recording, and beyond that the artists and their soul and passion. It's all a singular line of continuity from one human's mind to another. So I can see the appeal in having transparent gear that gets out of the way. However we have to consider transparency for what it really is IMO, and that isn't necessarily neutral or "flat." In fact something neutral can be distracting, what I like to equate with deafening silence. The lack of something can be a distractor. A lack of coloration can itself be coloration. I think true transparency is, at its most basic, a connection to the music. I can see in that sense how the Piano Fortes can be transparent for you. For me, they simply aren't.
Hmm. I really didn't intend that to be a cliche "we all hear differently" post. I actually think our hearing is more similar than not. I think our tastes are what differ, what our tolerances are and what we are used to, which can directly effect what allows us to connect to what we're hearing. It's that old "listening to gear versus listening to music" thing.