Background
 
I bought the ZMF x Vibro after owning the HE-400 for about a year. While I loved the HE-400's bass, my ears just got tired of the whole thing after a while. It was fatiguing even after I EQed the treble down by about 12 Db at 10KHz. I wanted to try a something warmer, and reviews of the Vibro had me intrigued so I bought them.
 
System
 
FLAC and 320 Kbps mp3 through Foobar to a Concero and Project Ember at home. I also used the Vibro with my Fiio X1 and Cayin C5 at work. I mostly listen to electronic, rap, rock, and metal in that order.
 
Now for the breakdown. I'll start with overall strengths and weaknesses and elaborate below.
 
Strengths
 
Gorgeous, luscious, detailed, enveloping mids
Warm but still very clean, not thick at all
Natural tonality. Very relaxing and immersive. These are headphones that present music as a coherent whole rather than as a sum of technicalities (which is not to say the technicalities aren't still done well)
Smooth, never offensive treble
Extremely forgiving--I've never heard bad source material sound so good
Open and airy. Don't sound closed. 
 
Weaknesses
 
Generally on the soft side. Too relaxed for aggressive music
Bass lacks ultimate extension and impact
 
 
OK, on to the specifics.
 
Bass
 
I'm a bit of basshead, I'll confess. The bass is adequate but a definite weak point (for my preferences at least). Drums and acoustic instruments sound great--the bass is very natural and it has an airy quality that lends realism to the timbre. With all three ports open and my Cayin C5’s bass boost turned on, the bass quantity is satisfying for EDM though I would still prefer more for rap (for reference, I think my P7s have a smidge too much bass with no boost/EQ, and I EQed the bass on my old HE-400 up by about 3 dB centered at 40Hz).
 
I tested the extension in Sinegen. Extension down to ~20HZ is there but pretty rolled off starting around 50Hz or so. This was especially noticeable when I tried watching Transformers. The Vibro just couldn't create the rumbling, subwoofery ambience of a movie like this. My P7's on the hand scrambled my brains with subbass on this movie. 
 
Quality-wise, the bass is a little soft and diffuse. It doesn't slam as hard as I would like. Again, this is not a problem for acoustic instruments but it is a problem for synthetic sounds. For my tastes at least.
 
Mids
 
The Vibro does mids fantastically. Synths wash over you in warm, shimmering waves. Textures have a soft richness to them, a holographic vibrance that's smooth without losing detail. Voices and guitars simply sound real. All of the details are there with an effortless realism. The mids really could not be better.  
 
Treble
 
I'm moderately sensitive to treble and the Vibro never offends. It's treble is smooth and gentle. While detailed, it does not stand out. I wouldn't go so far as to call it dull, but it does lack some energy compared to brighter headphones. These are on the dark/ warm side after all. The upside is that I can listen to the Vibro for hours without any fatigue, even with poor quality recordings. The treble is very forgiving. I personally loved the treble but I could see some wanting more. 
 
Detail retrieval

Detail retrieval is top-notch. Easily a step above the HE-400. The Vibro doesn't spotlight detail the way brighter headphones do, but if you listen closely a lot of information is there. I found myself hearing things I had never heard before with the HE-400.
 
Soundstage and Imaging
 
I was a little concerned about the soundstage after ordering these. They were my first high-end closed headphones. I was not disappointed in the slightest. The Vibro simply doesn't sound closed at all. The soundstage isn't the widest in the world but it's plenty deep and instruments have room to breathe. Separation is excellent, though the Vibro doesn't have razor-sharp layering like the HE-400 did. Rather, imaging is more diffuse. The layers blend together without ever sounding crowded. Distance-wise, the Vibro is neither upfront nor distant. Its soundstage is presented in a very natural way that doesn't call attention to itself or get in the way. 
 
Genre Appropriateness
 
I listen mostly to EDM with some rock and rap thrown in. The Vibros are great for rock. I don’t really care about subgenres so I’ll just name some bands that sound great. The Cure, New Order, Fleetwood Mac, The National, The Decembrists, M83, Tool, Isis, and many others. Guitars, synths, and vocals are all wonderful, and the bass presentation does well with drums and bass guitar.
 
EDM-wise, the Vibros do fantastically with lighter poppy stuff like Neon Indian, Baths, or Chvrches. But when it comes to music driven by the bass like Recondite, Com Truise, or Crystal Castles, they fall short. The pounding beat just doesn’t grab me the right way; the sense of drive is lacking. And while the Vibros do synths in a gorgeous, shimmering way, they can’t give them that ripping, buzzing, sharp texture I enjoyed from the HE-400. Likewise, rap just doesn't have enough weight for me. 
 
For both EDM and metal, it's worth noting that the Vibro's forgiving nature is a major asset. It gave new life to recordings that I generally skipped over with the HE-400. I felt like I was rediscovering some of my old favorites that hurt my ears before (Wumpscut, looking at you). 
 
Ordering, Customer Service, etc.
 
Zach is an awesome guy to deal with. I can't emphasize that enough. He is attentive to emails and you can tell he cares about his customers and his products. He was a total stranger to me when I ordered the Vibro but I moved to Chicago shortly afterwards and met him in person. Getting to know him only cemented my initial opinions. He loves what he does and he'll take care of his customers. He even gave my Vibros a touch-up/ retune before I sent them on to their next owner. Try getting that level of service from a big company!
 
Overall

The Vibros are fantastic headphones. They are both warm and clean, detailed and forgiving, lush and natural. If that sounds like contradiction, it’s not. The Vibro is able to have all those qualities at once by having a relaxed and soft signature. It’s a very pleasing signature, don’t get me wrong, but for my tastes it doesn't work for everything. I wish I could have kept it, I do miss it sometimes, but for me it's too expensive to keep around as a sometimes headphone. 
 
 

Comparison to AKG K340

Just for fun I’m going to add a brief comparison here. The K340 and the Vibro are maybe a little similar in that they are both closed headphones that tend toward the warm, musical side. However, the similarities end there.
 
The only thing the K340 does better than the Vibro is treble. The K340 has an effortlessly fast, detailed treble. It’s clearly brighter than the Vibro’s treble without being that much harsher. It is harsher, but that’s only because everything has harsher treble than the Vibro.
 
The K340 has rich mids, but they have a kind of weird recessed coloration. At least my pair does. The Vibro’s mids walk all over the K340’s by a longshot.
 
I’m going to call bass a tie. I EQ the bass up on both, but my bass EQ for the Vibro is actually 2-3 dB higher at the peak. Similar shape/ bandwidth. I would go higher with the K340 but it muddies up the sound too much. Anyway, the Vibro has better extension, speed, and detail but less body and impact. The K340 is a bit more solid but also slower.
 
Overall, The Vibro is the better headphone. It’s more cohesive and natural. 

Background and listening experience: I bought these as a complementary headphone for my ZMF Vibros. I wanted something open and I've been curious about Audio Technica for a while. In the past, I've owned the MDR-V6, DT-990 Pro, Q701, and HE-400. Listening was done straight off my laptop as well as through my Concero to Ember desktop setup. 

Overall: I'm very pleased with these headphones for the price. I would not hesitate to recommend these to anyone making a first headphone purchase, as long as you are not looking for monster bass. The fact that they can reach 99% of their full potential without an amp makes them a particularly practical choice. The AD-900x is a clean, open sounding headphone with natural timbre, relatively unfatiguing treble, and a moderate but still satisfying bass response. I particularly enjoy it with ambient, IDM, and all sorts of folk, rock and metal. I don't listen to much classical or jazz but I think it's well suited to those genres too. 

Bass: The bass is satisfying for most genres aside from rap and bass-driven EDM like dubstep. It's clean and tight with excellent impact and decent though somewhat rolled-off extension. These headphones put more emphasis on percussion and snap than on rumble. Drums and other acoustic instruments sound fantastic. Note that I am using a wide-bandwidth EQ bump of about 2 db centered at 35hz--this really brought the bass up from meh to satisfying for me. 
 
They can certainly reproduce sub bass but it falls back in the mix and doesn't give the music as much foundation as, say, the HE-400. The Q701 digs down maybe a tiny bit deeper, but the AD-900X has substantially better punch and impact. It actually comes pretty close to the HE-400 in terms of punch but doesn't have the same effortless, visceral sense of texture.

Mids: Fantastic. The mids are slightly forward and have a natural timbre. Acoustic guitars, voices, etc. come forward in the mix, but not too forward--they are emphasized but not enough to throw the balance off IMO. 

Treble: These headphones are bright but not excessively bright. I find the treble less fatiguing than any of the other headphones I've owned aside from the ZMF. After EQing the 9khz region down by about 2 db, the highs are smooth and sweet. Sibilance still rears its head occasionally on bright material but it is mild. 

Soundstage: Excellent. These have a very wide and deep soundstage, but they don't sound distant like the Q701 either--some sounds are close to you, others are far away in the background, others are in between. I'm still not sure if I know what "airy" sounds like, but these have a very open, spacey kind of sound. Diffuse is a good word. 

Imaging: I wouldn't say the imaging is bad, but these headphones have a unique approach to imaging. The sound is very diffuse--you can tell that different parts of a mix are coming from separate locations, but at the same time, the exact location is not clear. For example, the HE-400s are much better at giving me those moments where it sounds like something is behind me and I turn the music off to check. AD-900X positioning is less precise and doesn't have those moments where it tricks you, even though it has a larger soundstage than the HE-400. 

Resolution and detail retrieval: I'm not sure what to think here. On the one hand, these headphones have revealed details that I've never noticed before, even through the HE-400. I think the huge soundstage helps there--it makes certain background noises very easy to pick out. 

At the same time, my biggest complaint with these headphones is that they are grainy and lack microdetail. They just can't render sonic texture and inner detail with the delicacy, richness, and subtlety of the HE-400, for example. Instead, sounds have a papery sort of flat one-note texture to them. A little hard, but not offensive or metallic. The sonic pixels just aren't fine enough. Honestly, though, I think the resolution is fine for the price and more or less on par with the DT-990 and Q701. It's just that, IMO, this is the primary area where the AD-900X falls short of higher-end models like the HE-400 or ZMF. 

Amping: These don't need an amp at all. They sound 99% as good straight from my laptop as they do from my Concero and Project Ember. They are very efficient, but I think this also has something to with the lack of resolution mentioned above--they just can't reveal everything the Concero picks up. 

Also, I didn't like them at all using the 0.1 ohm output resistance setting of the Ember. The bass was anemic. They sound much better with the 30 ohm setting (the sound at this setting is similar to my laptop). They sounded fine through my phone as well. 

Comfort: The AD-900X is supremely comfortable. I can barely feel them on my head. It seems like the 3D wings don't do much and the clamp is the main thing keeping them on. Since they are so light, I don't even notice the clamp after a few seconds though. 

Build quality: OK but not great. They are made from plain matte plastic and they seem to be put together well enough, but their light weight combined with all the swiveling parts also makes them feel a little rickety. The Q701 is almost as light but its construction is more solid and its materials feel more premium. 

I am writing this review of the Grado PS500e as a part of the tour program sponsored by TTVJ. Thanks to Todd for giving me the chance to hear these great headphones.

I am chiefly comparing these to my current reference, the Hifiman HE-400. Listening was done with both my laptop and my Concero DAC + Project Ember amp, though it was mostly with just my laptop as I thought it sounded better (much to my surpise). In the past, I've also owned the AKG Q701, Beyerdynamic DT-990 250 ohm Pro, Sony MDR-V6, and others I'm too lazy to list.

Overall: These are a great pair of heaphones. I really like them. They are smooth, fun, engaging, not fatiguing, and resolving while also being very forgiving of poor source material. My music library is all FLAC, but these made even Youtube videos completely enjoyable. I haven't heard any other Grados before, so I don't know what the "Grado sound" really is, but I'd describe these headphones as generally warm, with a full bass response tilted towards the midbass, a slightly forward midrange, and sweet highs. These are NOT what I'd call Hi-Fi—they don't pick the music apart. Rather, they bring it together into a cohesive and natural whole.

Bass: I was surprised by the bass response at first. It's very rich and full. The extension doesn't dig quite as deep as my HE-400, but it's hardly a problem. Rap and electronic music were both a lot of fun. The bass is detailed and fast, but not really tight. There is a sense of body, with harmonic overtones and trailing edges being well developed. Compared to the HE-400, the Grados have more midbass emphasis and the Hifimans' emphasis on the leading edge of notes gives visceral synthetic textures more bite. The Hifimans also go lower. Nevertheless, the Grados present a greater sense of bass pressure overall. I also noticed that their bass presentation was very sensitive to the precise way they sat on my ears.

Midrange: Fantastic overall. My only quibble is that the upper bass can bleed into the mids on bass-heavy tracks. Otherwise, the midrange is spot on. It brings guitars and voices forward, it has a very natural and even tonality, and there is little to no glare in the upper mid presence region.

Treble: Pretty well balanced. There is plenty of treble resolution, but it has a smoothness that makes it less fatiguing than most of the other headphones I've owned. The last bit of extension and sparkle isn't quite there, but that's okay.

Soundstage/ Imaging: The soundstage is small, but it doesn't feel cramped or congested. Intimate and dense are better words.  Imaging is decent. The Grados can't replicate the Hifimans' super sharp 3D layering and positioning, but instruments are still clearly separated—I can pick out individual instruments easily. However, this is more a function of resolution than a truly "spatial" sort of separation, if that makes any sense.
Detail/ Resolution: Fantastic.  The Grados are just as revealing as my HE-400s, but they are less in your face about. Details blend together into the whole rather than starkly jumping out. Due to their cohesive, dense, warm presentation, the Grados reveal less black space than my HE-400s. The HE-400s present music as if you are in an exceptionally quiet and damped (maybe too damped) room; the Grados present music as if you are in a small basement bar with heavy brick walls that contain the sonic pressure of live music.

Amping: The PS500es are exceptionally easy to drive. Most of my listening was done on my laptop in the 20-50% range. The Project Ember was massive overkill and the Grados revealed a lot of hiss from it. I could also hear some hiss on my laptop, but not as much.

Comfort/ Design: These headphones are supra-aural and they can make my ears sore after a while. They are very light however, so it's not that bad. The build quality is good, but one that bothered me is that the cables aren't really fixed--you can't pull them out of the cups but you can push them in. I heard some fuzzy rattling sounds occasionally, and I had to pull the cables tight to fix it. The cable is also very thick but still flexible.  

Closing: As much as I like these, I don't think I'll be buying a pair. My HE-400s give me more of what I'm looking for (precise imaging and separation, deep, textured bass, and a generally clean sound, at the expense of some harshness and fatigue) for almost half the money. However, that's purely a matter of taste. These are excellent headphones. 

Context: I primarily listen to music through my setup of FLAC -> HRT Music Streamer II -> Little Dot Mk3 -> Hifiman HE-400. I bought these primarily to use at the gym with an ancient RCA mp3 player that only has 512mb, because expensive gear and sweat don't mix IMO. These setups fill completely different roles, so I obviously don't expect them to compare, but I think it's important to clarify that my reference system costs about 40 times as much as my gym system. Note that my ratings for value, audio quality, design, etc. in the details section are in absolute terms, not relative to the price. 

Now, on to the NE-600X. I am extremely pleased with these for the price. I have gone through a number of lower-end IEMs over the past few years (mostly Sony and Altec Lansing UE clones, don't remember the exact models), and these have been my favorite. I have no reservations recommending these given the price, as long as you like bass. 

Sonic Balance: Bass-heavy, but not V-shaped. The midrange and treble are balanced and relatively smooth. These are dark and laid-back overall. 

Bass: The bass on these is very emphasized. It certainly resembles the typical boomy, basshead bass found in many headphones in this price range, but I find that it's a little better. The bass never gets farty or sounds like it's clipping -- it's just big, wet, and powerful. It obviously doesn't have the extension or speed that I expect in an expensive, full size pair of headphones, but it has a very satisfying sense of weight, pressure, and power. Definition is above average for bassy IEMs in this price range, but nothing fantastic. It is slow in absolute terms, but reproduces heavy textures well. Poorly mastered rap (i.e. most popular rap) sounds more fun on these than on my $400 planar cans, because these put enough oomph in the midbass to mask the lack of subbass in the recording. 

Midrange: Not much to say here. It's fine. The bass can bleed into the midrange at times, but the NE-600X is better in this respect than other IEMs I've had. Midrange detail is above average at this price level. 

Treble: The treble is smooth and not fatiguing. I don't hear any sibilance or harshness at moderate volumes, but there is not much air either. I appreciate that the treble isn't boosted for the sake of artificial detail, but it is on the dark side. The treble is the first to deteriorate when I crank it however -- it starts getting splashy, uncontrolled, and a little harsh. 

Soundstage: I was most surprised here. Obviously the soundstage isn't big, but it is very natural for a cheap IEM. There is layering and separation between different instruments, so that I have a sense of how different instruments are located in the room. Vocals are clearly presented in space, and samples and other effects are positioned where I expect them to be (after hearing the same song on my reference set up).

Detail: These don't reproduce much, if any, low-level detail, but textures and tones are resolved in a natural way. 

Build: These are very solid. They are all plastic, but not flimsy in the least. A little more heavy than other IEMs, but not enough to be annoying. The cable connections to the earpieces and to the jack feel very secure, and the jack is at a right angle. I also appreciate the flat cable, as it resists tangling effectively. The cable doesn't feel like it could ever snap in two, unlike some of the delicate cables I've seen on Sony IEMs. 

Fit: This is very personal, so I won't complain too much, but I have a hard time getting a seal and they tend to fall out more easily than other IEMs. This is probably due to my ear shape, but the insertion is very shallow, which doesn't help. I am usually aware of them in my ears, so they don't disappear completely. However, they never hurt. 

Isolation: The insertion is shallow as I said, so these don't completely cut out the outside world. However, they do block enough noise that I don't hear normal low-level background noise when the music is off, and probably wouldn't notice someone trying to talk to me if music were playing. They aren't going to *completely* isolate you from loud background noises like traffic on a city street, however.