I received my replacement pair. Fortunately no manufacturing defects on the second one.
I have discovered something about these headphones. The Massdrop pads are a problem sonically.
I replaced them with the stock Koss ESP/950 pads and the improvement is significant. The Koss pads place the drivers closer to the ear, and the foam is softer than the Massdrop version, which enables the pads to conform to and seal to your face better.
The result is a much improved bottom end and a fuller overall sound.
I've revised my rating upward based upon this. I struggled whether to give it 4 or 5 stars.
The case for 5 Stars:
These headphones resolve more detail than any other I own.
Separation of multiple voices amongst double-tracked harmony vocals is superior.
Acoustic instruments are more lifelike
They are very comfortable.
I don't know what else at $500 offers this degree of realism, genuine detail, and relative neutrality.
After listening to these for awhile, when I revert back to my other favorite phones, I notice flaws and colorations that I previously didn't take much notice of.
I don't see myself ever getting rid of them.
The case against 5 stars:
They don't sound spectacular on as many different recordings as some of my other phones. How good they sound is more dependent upon the tonal balance of the recording.
They tend to seem a little bit midrange-focused.
They tend to start breaking up a bit in the bass at loud volumes sometimes. I suspect this is a function of the amp/energizer. Most people probably won't get there, but sometimes I like it loud.
They lack the bottom end slam of planars (an inherent characteristic of electrostatics)
I still find some of my other headphone/amp pairings in the same total price range more subjectively "fun" to listen to overall, such as HE-400i & CTH, or HE-400S & MD THX AAA.
You need to "modify" the cans with the ESP/950 pads to achieve best results.
When I ask myself the question "If I could only own one $500 headphone/amp combo, would it be this one", I have a difficult time answering that question.
So, I'll let you be the judge. If you can afford to spend $500 for a headphone that isn't endgame, but does offer a wonderful flavor that you'll want to taste frequently yet not every meal, then I highly recommend them. They are also my new go-to headphones for studio work to cross-check mixes and masters.
I'm underwhelmed by these sonically. Based on reviews I've read, I was expecting great sound and poor build quality. My impressions are somewhat reversed. While the build is lightweight plastic and indeed nothing remarkable, fit and finish is good, there are no rough mold edges, etc. I think there may be a quality control problem in manufacturing though, as mine arrived with all of the screws that hold the Energizer case together completely missing, and the back panel of the Energizer fell right off as soon as I removed it from the packaging, exposing the inner circuitry.
Sonically, I'm not all that impressed. I've never owned electrostatic headphones before, but I have owned a pair of Martin Logan Aerius I speakers for 21 years, so I'm familiar with the e-stat sound signature. I'm not a basshead, but I definitely think these are lacking in bottom end and a bit thin sounding unless I'm listening to music mastered with a lot of bottom end. Stuff that sounds excessively bassheavy on other cans sound like there's almost enough bottom end on these. There is a bit of high end sparkle, but there is a weird tonality to sharp esses that doesn't seem right. I honestly prefer the sound of my Hifiman HE-400S to these by a wide margin. Where the Koss shines is on detail in strings and other well recorded acoustic instruments. There is a remarkable realism and natural texture to a bow moving across a string that is more lifelike than any other headphone in my collection of many.
Lastly, my right earspeaker sounds thinner than the left, with less max volume. I think if you listen to classical and maybe some good traditional jazz recordings, a properly assembled and functioning pair might be a worthy addition to your collection. The build quality is better than I expected (except for all the missing screws), but I was anticipating much better sound.