They're different, significantly, in many ways.
Yes, I'm curious about the OK1 too.
I haven't written an official review of the PFE yet, but I can run down on a few differences.
First, they sound quite different from each other. The Denon has a more effortless sound because it is a very efficient design. The sound is "real" but in a dry. The Phonak isn't quite so "real," but it also doesn't color and make it sound wrong either. Both need EQing to sound their best. The PFE makes good use of an amp if you've got one. The Denon has zero need of additional amping. The Phonak is much more output capable, think 3x. Because of this, the Phonak can better represent a dynamic range from quiet to loud where as the Denon really needs a quiet environment to do similar. Little sounds are more noticeable with the Denon, like clicks or a finger sliding across a guitar string. It's not that it's not there on the PFE, but it's presented differently.
Bass, PFE wins by a mile. The Denon is critically flawed in output capability and bass response suffers a good deal at anything above quiet to medium levels. The Denon doesn't play as low and is poorly controlled making for a sloppy experience. It also doesn't extend in frequency as far as the PFE. The PFE is clean, exended, and with great big woofer type energy.
Midrange, both are kind of on par, neither doing anything wrong, but both do present sound in different flavors, the PFE a bit more analytical and ultra clean. The decay of note is shorter. With the PFE you get lots of crispness and cleanliness but without the depth of note. Without EQing, the Denon's treble and bass overshadows the mids some making they sound more recessed then they should be. However, the Denon has better body in note and has a better representation of breadth of note. This is a unque aspect I liked much about the Vibe and the ER4S shows this exceptionally well too. Not enough breadth creates a shortended, analytical presentation, squeaky clean but less real.
Treble, the Denon wins. The PFE simply needs EQ work to sound as good as it should be. The highs are strong and clean though. Highs are there but not the best in balance. The Denon also needs some EQing, but it takes less work to fix, and once the highers are brought down to midrange level, the treble is just as good as the mids. Both are good really, but the Denon just needs less work.
Overall, the PFE has more energy, especially when amped. The Denon offers more body in note. They are pretty different in sound from each other. The PFE is very clean sounding and with a lot of energy. The Denon a little drier and fuller. I would lean more towards the PFE with normal use because it's easier to work with and offers more overall capability. The Denon is output limited and needs more EQing to get to a really good state. The PFE can be used as is with the black filter, but can improve with some top end work. The PFE is just easier to enjoy out of the box.
Right now I do feel the ER4S beats both, but pricing is a step above either. The ER4S lacks the intensity of the PFE and the effortlessness of the Denon but is a whole lot better balanced everywhere then either. Frequency response is really good, attack and decay of note is "correct" offering great breadth and a realistic sound. The PFE has a more robust bass response, digging deeper but the ER4S is fuller in note, the PFE giving intensity and impact, ER4S giving body. The ER4S is more like the Denon but less effortless sounding but a lot better balanced in frequency response. The ER4S is cleaner. The ER4S is more spacious.
I would consider the PFE and ER4S a step above the Denon. I'd LOVE to see Denon retry the IEM game with a new, better headphone. If they could provide greater SPL capability and a flatter frequency response, I'd love to hear it. As the Denon stands now, it's simply too flawed, enough so where it's intended use becomes limited.