These are a great pair of cans at the $169-odd price point. Despite claims of higher efficiency (and thus satisfactory performance with low-power-output devices) I would still recommend an amplifier, as I noticed some "confusion" when playing music even distribution of instrumentals across the frequency range, e.g. base was good but guitar was suffering.
If you want a good album for a pair of open-back headphones, check out Hans Reichel's "Yuxo (A New Daxophone Operetta)". A particularly good track would be "A Life Without Lychees" - it has some great texture.
On to a real pain point - I would actually spend more time writing about audio quality if it weren't for the fact that the headband gimbals have me so pissed off that I am actually bothering to write this review.
My pair of headphones has an uneven amount of tension on the Y-Axis joint (where the yoke connects with the sliders). In more specific terms, this means that (in my case) the left cup may be rotated with less effort than the right cup - and noticeably so. With the cups removed, the left yoke will freely (e.g. without interaction) rotate, while the right yoke is incredibly stiff and requires interaction in order to rotate. This concerned me, as many people have experienced a scenario where the threaded metal insert(s) may come free from the plastic driver housing. I do not want the increased tension from the right rotational joint to put excess stress on the inserts in the right cup, thus unseating the insert.
Now, in inspecting the right yoke, I noticed that the rotational joint is built by sandwiching the yolk between a series of plastic spacers, which are in turn held in place by an un-keyed flanged bolt threaded in to blind nut with very shallow flat-head keying. The tension required to rotate the yoke is thusly proportional to the amount of tension on the bolt/nut assembly. One might adjust this assembly were it not for the fact that threadlocker (almost certainly loctite red or equivalent) as been (mis)used in the bolt assembly. One might also hope to apply enough tension as to break the bond of the thread locker were it not for the fact that care had obviously been taken to select the softest nut (or bolt, if that's the keyed end - it's not clear) apparently available on the market. I have seen a lot of cheap bolts and screws. I have never seen one that was so ready to strip out as this one. Truly incredible. Applying the slightest amount of force chewed up the head of the bolt/blind nut quite heavily (and I should add that I used a near-perfect fit, full-width flat-head bit). Looking at the metal, it's a slight rose gold color and looks a lot like brass — though I would be very surprised to see brass used here. My guess, based on the chewiness of the metal, is that these are either tin or aluminium. Additionally, I take issue with the use of thread locker here, as it really should be reserved for use in scenarios where one is concerned about a screw/bolt/nut freeing itself due to such things as vibration, etc... If HiFiMan were concerned with the possibility of the yoke rotation/retention assembly coming loose, they should have opted for a rotating slider assembly, as is used in all
Luckily, I don't really care about the cosmetic appearance of the headband - but someone who may should be wary of this issue.