Now that I've had several days with the HP-3, I figured I would make a mini review and share good and bad that I've found with it.
Build 9/10
With no surprises to anyone, the build is pretty great. The only flaw that I have found so far is that my set, when the cups are turned to lay flat, the right earcup isn't level with the left, suggesting that the curve of the headband is symmetrical. This is annoying to me but I never lay them flat so it's not very noticeable otherwise.
Fit & Comfort 4/10
This are where my biggest gripes with the headphones are. While the design looks nice, it definitely isn't designed with comfort in mind, especially the headband. The narrow width, small amount of stiff padding, and small contact area across my head was the perfect storm me. The lower clamp force and heavy construction just make things worse. After only an hour the pain on top of my head becomes bad enough to distract from the music.Also, because of this discomfort I would constantly fiddle with the fit, trying to reposition it to a more comfortable spot, usually in vain.
So if I was going to keep them, I needed to do something about this. I could have used Dekoni Nuggets as others have suggested but I really dislike the way this looks and the thought of using adhesive on a headphone this nice makes me squirm. So instead I have made a leather suspension strap from a piece of cream colored leather, pictured below. I would have liked to use the same caramel color leather that's used on the pads and headband but I knew that I would never find a piece that was the exact right color so I went with a contrasting color instead. While this greatly enhances the comfort for me, this brings to light another problem. In order for these headphones to fit my head properly when stock (I have a tall head and my ears are lower set than most people's are) I have to set them to setting #6 on the headband slider (which is the last marked number but they do extend slightly more than this), so in order to have enough room for the strap to keep the headband off of my head I only had a 1/4 inch of room to play with. So now I have the strap I have to fully extend them and they are barely big enough. So if you have a big/tall head with lower ears, this headphone might not fit you at all, let alone leave enough room for a comfort strap.

The earpads are on the harder and shallower side but aren't terribly uncomfortable since they are genuine leather and so breathe decently well. Although they aren't ideal with glasses. I find them a lot more comfortable without my glasses on, which is typically something that doesn't bother me this much with other headphones.
One minor annoyance that I wanted to mention briefly is that because of the rear entry cable design on the yokes that combined with the lower clamping force can cause using these headphones while lying down problematic, usually making the use of a pillow impractical since it pushes the headphone forward too much.
One last thing to mention in this section is that my pair doesn't have the channel imbalance that I thought it initially had. The fiddly fit in a combination with very sensitive imaging and very slight channel imbalances from that even the best analog volume controls have, I thought it was a driver mismatch, when it wasn't.
Soundstage 6/10 & Imaging 10/10
Soundstage is on the smaller side of things. It's not quite the in-your-head experience of the HD6XX but it doesn't really extend more than a few inches away from the cups. This ins't terribly surprising since these really aren't open headphones despite the looks. They are on the more closed side of semi-open and when you are wearing them they block about as much sound as a poor-isolating closed-back.
The HP-3 have some of the best imaging that I have heard. It is not only excellent side to side but also back to front. A good example of this is "Intro" on Yosi Horikawa's
Vapor album, the bass actually circles around the listener while most headphones can only make it sound side to side. "Letter" from the same album is another good example of good imaging with smooth transitions from left to right while he writes across the page. Headphones with worse imaging tend to lump it into far left, middle, and far right, making this song more jarring. The only other headphone that I have that can compete with the imaging of this headphone is the DT1990 but given how much more analytical that is, it's impressive that the more "fun" HP-3 can hang with the best.
Dynamics 5/10
This was one of the bigger disappointments for me. The HP-3 has a powerful sound but it really phones it in when it comes to dynamics. I was never surprised when a crescendo occurred or when a build up pays off. "Mountains" from the
Interstellar soundtrack is a good example of this. When the horns and organ finally come in, it doesn't shock or surprise with the volume difference."Take Five" from The Dave Brubeck Quartet's album
Time Out is another example. The drum hits starting from around the 2:30 mark do have the same amount of dynamic impact other headphones have. For a music-focused high-end headphone this is category I thought these would own, but alas, that's not the case.
Sound 8/10
Overall, these sound good, but I don't think they sound good enough to deserve their original listing price of $1,200 when there is so much great-sounding competition lower in the price bracket. For the $700 I paid, I feel like the price-to-performance ratio is pretty close to in sync, which is honestly disappointing. That being said, these sound very good with some caveats.
Bass
These definitely have the biocellulose character to the bass although it's much more tame than the Fostex TR/H-X00, probably due in part to the more open design. The bass extends pretty well, hits pretty hard (although the lack of dynamics takes out some of the potential impact here) and is pretty even across the spectrum although it does bleed a bit into the mids and can have extra reverb depending on the frequency. "Letting Go While Holding On" on Nine Inch Nails' new
Ghosts V: Together album is one of the tracks I noticed this the most. A minor gripe overall.
Mids
This is a section of the sound that's a bit recessed but still sweet sounding. Almost like the music is being performed in a wooden concert hall. The HP-3 isn't super v-shaped but it's definitely where it leans. Because of this mids-recession, songs can sound kind of hollow. This effect is much less noticeable when it's being amped properly. They are easy to drive but picky about the power you give them and these definitely benefit from cleaner sources.
Highs
I'll be blunt here, the HP-3 can be strident and sibilant in the highs. This isn't particularly bothersome to me but I own 5 pairs of Beyerdynamics so take that how you will.

This is one area where the headphone has a lot of detail but it's taking a bit of the brute force approach here with the recessed mids and elevated highs to get there.That being said, it's effective. I have heard details with this headphone that I never heard before which is becoming rarer the longer I'm in this hobby and get better gear. One interesting area that I notice more detail is when reverb is applied to a singer's voice it's super easy to pick the effect out. "Send My Love (To Your New Lover)" on Adele's
25 album is an example where the effect is clearly audible.
Conclusion
If I had to sum these up in one word I would say "rich." From the build to the sound it has a very rich character. The HP-3 is an interesting headphone. It's more v-shaped than I thought it would be, honestly. A lot of high end stuff tends to shy away from this tuning which is a fine sound when it's done right. Another thing, I mentioned this above but what these are driven with does make a difference despite them being "easy" to drive. Getting volume is not problem but getting good sound is another problem entirely. My favorite pairings have been the IFI ICAN SE, and THX-789. I haven't tried them balanced since they don't need the power but I might try that later although I'm doubtful it will make a huge difference.
I do find it unfortunate that they went a little too much form-over-function especially when it comes to the headband but this doesn't seem to bother everybody so it could just be my head that it doesn't like. If anyone is interested in making their own comfort strap, I would be happy to scan and post the pattern here, just let me know. It was honestly pretty easy to make once I figured out what length would be enough to keep the headband off of my head while also not making the headphone too small for me.
Stay safe, and happy listening everyone!