I just wanted to give an update after about 10 months of ownership.
Firstly, the sound is just fantastic. I won't go into details, as many others better equipped to describe it have done this already. All I will add is that it is every bit as good as claimed, period. Based on sound quality alone, these are one of, if not the best bargains for headphones in this tier and sound category.
However, there are two serious problems with these phones - comfort, and construction quality. The latter leads to to terrible clunking and popping noises in the cups. I've done some workarounds that have resolved 90-95% of both of those problems. Unacceptable on a pair of headphones in this price range in my opinion, but also worth it if you've already bought them and can't return them.
Comfort:
Although some report that out-of-the-box comfort is adequate, it was problematic for me, like many others. While I quite like the construction and material used on the ear cups (not too soft, not too firm), the inner circumference and volume of the ear pads are a little too small, so my ears pressed against the foam inside and against the edges of the cups. I alleviated 90% of this problem by adding a piece of cable on the inside of the cups to push it out a little, giving more volume. This also increased the sound stage a little, with no negative impacts on sound quality IMO. I tried fitting Brainwavz HM5 regular pleather pads, but they completely ruined the sound.
Secondly, the headband is poorly padded and designed, and consequently puts too much pressure on the very top of my head (again, like many others). I got around this by securing an aftermarket pad under the band using velcro zipties. Not the prettiest mod, but it works and looks reasonable.
I detail these mods
here, except that I now use velcro ties instead of the electrical tape I was using to temporarily fasten the padding to the headband.
Construction Quality and Noise
For myself and many others, the headphones begin to make clunking, creaking, and popping noises in the cups after a few weeks of use. It seems to come from the hinges inside the cup, where plastic rubs and sticks and suddenly releases while bending. Because the cups are essentially empty and made of plastic this amplifies the sounds, making subtle little creaks heard externally sound like a bombastic warzone while wearing the headphones. Simply turning your head or leaning slightly backwards or forwards released a cacaphony of noise. At first I accepted this, but it continued to worsen with time. Eventually, I found myself avoiding the headphones and unable to enjoy listening to them.
Fortunately, there is a way to mostly fix this. It involves disassembling the cups, lubricating the internal hinge, and putting some putty/tack on the back of the hinge where it seems to rub against a plastic cover that separates the driver from the inside of the cup. This repair has resolved 95% of noise problems and made wearing these a pleasure again. 100% worth the effort for me.
NOTE - You CANNOT fix the problem by applying lubricant to the external arm near the hinge - it simply won't reach the actual hinge and won't prevent plastic rubbing on plastic. Under no circumstances should you spray an aerosol-based lubricant into the external hinge area to reach the inside, as you risk damaging internal electronics and the drivers by exposing them to solvents and lubricant.
Original (?) simplified instructions for the mod are detailed
here and
here.
Here is a different option, putting electrical tape over the internal hinges. I suspect that the tape will stretch and wear out over time, requiring a "refix later". I feel more confident about the lubricant and putty/tack trick - the putty remains pliable for years.
This is a risky mod and I recommend you don't actually do this unless you have a steady hand and are prepared to have the headphones resoldered. The wires that link the driver to the cup are extremely thin, short, and easily broken.You will basically need three hands to really do this properly, but it is possible with two. For what it's worth, I have shaky hands and very little experience with electronics, and have accidentally damaged wiring in other headphones before. Worked out for me this time, but I wouldn't want to open them again.
If you are prepared to tackle this, here's my method. Unfortunately, I was unable to take pictures while doing this and do not plan to open the cups up again to do that.
I am in NO WAY responsible if my instructions end up causing damage to your headphones!!
Fixing the Noise Problems
Background:
The object is to lubricate the hinge connecting the headband to the cups and to put some pliable material over the outside of the hinges to reduce plastic-on-plastic contact.
The ear cups are composed of a plastic outer shell, a plastic frame that makes up most of the body, and two plastic plates nested inside that keep the drivers separate from the cups. A thin wire connects the driver on the uppermost driver plate to both the cable end and the headband hinge. It can and will break with only the slightest amount of tension. Under no circumstances can you put tension on this wire. Unfortunately, you cannot complete this modification without manipulating the wires somewhat. Move wires very slowly and avoid sharp bends and twists.
What you need:
Micro-screwdriver set with multiple phillips-head sizes. The smallest head is very small
Real silicone lubricant. I used some silicone plumber grease but treadmill silicone oil would probably work. Grease sticks better but is harder to apply. DO NOT USE mineral oil or petroleum based greases, they might dissolve the plastic.
A piece of tape to help remove the inner plastic piece.
Blu-tack or some similar tack putty. Used to fix posters to walls etc without leaving adhesive residues.
Lots of light, preferably a headlamp.
Terms:
I will refer to the following for orientation purposes
-Driver plate: uppermost plastic plate in the cup containing the driver
-Middle-plate: a thin plastic plate under the driver cup.
-Cup body: a plastic frame making up the body of the cup in which the upper two plates nest.
-Cup shell: the outermost curved plastic that forms the outside of the cups.
-Band-end: side of the cup where the band/swivel attaches
-Cable-end: side of the cup where the cable plugs in
Image with labelled parts
Original photo courtesy of innerfidelity.com
Great pictures of internals by user
seethelight
Method:
-Remove the earpads by gently pulling off the cups along the edges.
-Unscrew the 4 screws along the four corners. These screw heads are a little larger than the second set you will encounter. This plastic plate contains the driver (driver plate) which is soldered to the side nearest the headband/swivel. Leave the plate in place for now.
-Underneath the driver plate there is another plastic plate resting in the cup. It isn't affixed with screws, just pressed in a little. It will need to be lifted using a piece of tape to help pop it out.
-To remove this middle-plate, very carefully lift the edge of the driver plate up about 1/4" at most at the cable end. Leave the piece of foam in the hole in the middle in place. Slip a piece of tape under the driver plate (cable end), affix to the middle plate, and gently pull up on the tape to pull the middle plate from the cup.
-Once loose, gently pull the remainder of the middle plate out of the cup and press up into the driver plate (there is a little knob on the driver plate that fits into a hole in the middle plate).
At all times, try not to lift the driver plate much more than a 1/4" from the cup to avoid tension on the solder points at the driver.
-Now you will need to unscrew the exposed screws at each corner from the cup body. Gently move the driver/middle plate at an angle to expose the screws one at a time. These have a smaller head.
-Once the screws are removed, gently fit the driver/middle plates back onto the cup body so it looks as it originally did. Then, gently pry the cup body (with nested plates) from the cup shell, lifting it no more than a 1/4" from the shell.
-Here, the wire that connect the plates and cup body attaches at the cable end of the cup shell. It is short and may be curved around and slightly under the circuit board near the cable jack. Although slightly more robust than the band-end wires attached to the driver above, these wires are also thin and easily broken.
-Carefully lift the headband end of the cup upwards, watching the wire. You may need to move the wire out from under the circuit board to get enough slack. Angle the headband end away from the shell, exposing the hinge.
The way in which the cup shell is resting in this
picture in this
post is risky IMO, so be careful. Also, remember that the swivels are spring loaded and the band under tension, so if you let it go it might move suddenly, putting tension on those wires!! This is where the third hand really helps.
-Check to make sure the screws on the hinge are tight. Some people found them loose, mine were already tight.
-Dab some silicone grease around the points of contact on the hinge. Gently rock the cup shell back and forth to work the grease into the hinge. Be mindful of the wires!
-Roll a little bit of blu-tack or similar tack putty into two pieces, each half to 2/3 the diameter of a green pea. Press onto the each of the hinge points outside of the main hinge as in the picture above (i.e. not on the part of the arm between the hinge points).
Now you do everything in reverse.
-Tuck that wire back under the circuit board if you pulled it out. Gently press the cup body and plates into the cup shell. There might be a slight resistance since you put putty in there but you shouldn't have to apply any real force for it to go back in. Watch your wires the whole time!!
-Separate the middle plate from the cup body again using tape if it is stuck back in. Again, be mindful of the wires as when disassembling, keeping the driver plate as close to the middle plate as possible and still being able to separate the two.
-Screw the cup body back down on all 4 corners and nest the middle plate back into it (slight pressure may be needed).
-Check your wires and make sure the foam piece nested in the middle plate below the driver plate is in place. Then put driver plate back onto middle plate and put screws in.
-Feed ear pads back onto cups. Start at one end then work your way down the sides to the bottom.
Now manually move the ear cups back and forth a bit. Notice that virtually all the subtle popping/sticking sounds are (hopefully) gone. Put them on and enjoy some music!!!
I also put a little grease at the swivels on the arms where the band extends, as it would sometimes catch a little and make a resonating thunking noise too. Note that it is the lower part of the arm that extends from the metal rod within - the rod itself is stationary. Take a good look to see which parts move and which don't so you can apply it effectively. Not sure how much worked into the swivel joints, a little silicone oil might be better but there is wiring in there so use it very sparingly.
Final Notes
It worked for me, and has saved me from selling my phones. I'm back to enjoying these as they were meant to be heard!! In future models, I hope Paul Barton addresses these significant problems. I'd certainly be willing to pay another $50 out of the box for better comfort and construction quality. Possibly more if the phones offered improved sound and/or function. For now, I'm very hesitant to recommend these phones as comfort is an issue for many, and it seems almost a certainty that you will develop these noise problems, which are considerable and only resolved by the method I outlined.
Many thanks to members
n05ey and bgentry for their methods and ingenuity. I hope my more detailed methods make it a little easier for others who wish to try and save their HP50 headphones.
My HP50 rarely gets any play time anymore especially for serious listening, not that it sounds bad, but my ATH M50 is just that much better. 
