Superlux HD-668 B

Available for right around $40, the Superlux HD668B is hands down my favorite entry level open back headphone. As some one who started out with a Beyerdynamic DT 990, my cost of entry to the world of audiophile headphones ran me right around $250. That was for both my headphone and the little amplifier I needed to power it properly. What makes the Superlux HD668B so amazing, is that it offers a similar sound as the DT 990, clean, clear and wonderfully detailed without sacrificing bass, for significantly less money AND without the need for an amplifier! 
 

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You can purchase here on Amazon for $39.95
 
 
 
Specifications

 

Drive Type: Semi Open Back
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Frequency Response: 10-30kHrz
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Sensitivity: 98 dB SPL/1mW
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Maxium Power Input 300mW
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Impedance:  56 Ohm
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Weight 7.8oz (222 grams)
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Cable: Removable 1M & 3M 
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Plug Adapter: 3.5mm to 6.5mm (1/4 inch)
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The Superlux HD 668B comes in very simple packaging. Cardboard box, with the brand and product name on the front. Specifications are listed on the back side, and the left panel has depicts what is included inside.
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Accessories include a 1M and 3M female to male 3.5mm TRS along with soft plastic cloth carrying bag. Given the cost, I'm very happy to have the addition of the adapter and two cable lengths! 
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The design is simple, reminiscent of AKG, and more importantly it fits almost every one. I've never personally had an issue with the wing system, though I've owned a few Audio Technica Headphones myself so I'm used to it.
 

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A detachable cable, and easier to power driver make it easy to pair the Superlux HD 668B with a variety of sources. I've enjoyed it with my LG V20, my Audio GD NFB10ES2 and even my New Nintendo 3DS. With each, the quality of sound and total volume was beyond sufficient. For music play back I've primarily used my HM 601, and I will be using that as my primary source for the sound portion of this review.
 
I also purchased a KetDirect 4.5ft BlueCloth cable terminated to a right angle 3.5mm, I happen to prefer a right angle 3.5mm for my needs and if you need something different you're free to upgrade and swap cables how ever you see fit!
 

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With a $40 cost, I'm satisfied with the build quality of the HD 668B, the plastic is sturdy with some flexibility to it.
 
 
 
 
 
 

 
The wing system bends and allows the cups to place themselves just over the ear, and the ear cups also flex and rotate to further help with the fit.
 
 
 
 
 
 

 
 
My only qualm with the HD 668B is quality control, sadly my first pair had a very loud resonance on the left driver from around 230-250hrz. I simply returned that one and repurchased another pair from amazon. Thus far I haven't had any problems with the replacement.
 
 
 
 
 
 
 
 
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On the plus side, the pads are also removable and easy to swap out! I've yet to change mine out, but doing so does offer changes to the overall sound, so for this review I'm using the stock pads.
 

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I find the stock pads to be plenty comfortable, even in the heat and humidity that's common here in the south eastern United States. Being semi open, the headphones do block out some central to higher frequency noise, things like fans and traffic. However you can still hear people speaking, vehicles and the rumble of heavy to light machinery. I've walked around my house while cleaning, or working with no issues. I've also spent some time at the gym with them, and while the sweat from the pads was problematic, the lack of noise was not. I was able to focus on my music, while still having enough audible awareness of those around me.
 
 

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Sound​

For the purpose of this review, I paired the HD 668B with my Hifiman HM 601.​

 
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The overall tone of the Superlux HD668B is Cold and Dry, I find it's presentation to be very Bright and the overall sound signature is a fun neutral. Measurements show that it has a slight v shape to it's frequency response.
 
I really love the bass of the HD 668B because it gives a wonderful speed and texture to a variety of drums. It even stays true to the power of an electric bass and the warmth of a stand up bass. My favorite moment with them, was finally getting to really HEAR the bass line in Metallica's The Shortest Straw, in addition to some intimate fret action added into one of my favorite bass solo's. Synth Sub bass lines are the only draw back to the HD668B bass presentation in my opinion. I enjoy the extra speed and hardness that the HD668B has to it's bass response.
 
The mids are very dry, however out of my HM 601 I found a wonderful timbre to many brass wood winds. Trumpets, the sax and even flutes all had a very life like presentation, trumpets maintained a metallic edge without sounding tinny. The Alto and Tenure Sax both had excellent body and bite. The bright presentation of the HD668B also presented the quick breath taken in-between each note. It is problematic with stringed instruments. Heavy Metal riffs had a nice crunch, but not real bite to them. Acoustic guitars, while having great timbre in the lower notes, had a very uneven sound overall and the same can be said for almost all stringed instruments. The harmonics of many of those instruments were lost in the overall presentation of the HD 668B.
 
Top end clarity is another pleasurable quality of the HD 668B, high hats, cymbals and the brightest guitar solos all have a wonderful sense of clarity and air. While some may find the treble to be too hot, or too bright, I really do like the shimmer that the HD668B brings, really makes you feel like you're walking on air.
 
Sound stage presentation is very wide, but lacks some depth. Left, right and center separation are excellent and though it lacks depth, the Superlux HD 668B holds you more or less equidistant from each instrument. On tracks with a very strong sense of depth it is audible, but it is by no means top of the line.
 
It's lack of depth is often overshadowed by it's amazing mirco detail or transparency, it's a headphone that will certainly add a sense of atmosphere and ambiance to your music. An let you really hear EVERYTHING.  With the Superlux HD668B the devil, is certainly in the details! Good or bad, the Superlux HD 668B brings detail, if it's captured, chances are you'll hear it. This is also true of the equipment that you pair the HD 668B to, my HM 601s warmth helped to improve the tone of the HD668B, it softened a very stiff bass, and helped balance out the low to central mids.
 
In conclusion, the HD 668B is an incredibly transparent and fast headphone. While it's a tad drier than neutral, it pairs very well with a lot of entry level source and amplifiers, so chances are you'll probably not notice a lot of what I have,  and best of all it's $40!!! So not only is it a total steal for this level of transparency, but it's also a fun headphone to own! There are so many ways that you make little adjustments to how the HD 668B sounds, from the cable you use, to the pads, to your source, to your amp, and if you own tubes I'd imagine it would be a blast to listen to tubes with! All in all, I highly recommend the Superlux HD 668B.
 

Comparative Music/Cinema Break Down ​

Click here for a look at my review process ​

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The Zephyrtine - A Ballet Story [ Fundacao Orquestra Estudio, Rui Massena]  #09 Dance of the Zephyrtine 

• Superlux HD 668B ​

  1. I really loved the width of the sound stage, but the lack of depth got to me.Often times I found I couldn't appreciate the soundscape as a whole it constantly felt like something was missing. With my HM 601 as a source, I really loved the tone of the trumpets, they had a nice edge to them. I found the flutes to have a good shimmer to them, along with excellent clarity and decay from the bells. The high hats and snares sounded excellent as well, with a good crisp sound. Though the big timpani drums sounded very boomy with no real deep bass body to them, the rolled off sub bass response was pretty obvious with this track. 
• Switching to the Nhoord Red V1

  1. The Timpani drums don't sound as boomy, or forward, and there's a nice low end rumble and natural body to them. I do miss some of the shimmer in the high end with the Nhoord though, it's a tad mellower than the Superlux HD668B. But the more balanced mids on the Nhoord Red V1 add a playful quality to the flutes, that was missing with the Superlux HD668B.
• Switching to the HE 4

  1. Finally some DEPTH, while both the Nhoord and Superlux both had good width, the HE 4 brings a more holographic or 3D sound scape. In addition, that extra space presents mirco detail a lot more naturally. It's not as IN YOUR FACE, as with the Superlux HD 668B, not to mention there are a few details that the HE 4 brings to light, that neither the HD 668B nor Nhoord picked up. 

Kind of Blue (2015 Mobile Sound Fidelity Labs Re-release) [Miles Davis] #01 So What

• ​Superlux HD 668B ​
  1. The double bass has a really cool, smokey mellow tone. Though, it sounds a little uneven, the lower notes overshadowed by their harmonics, and the upper mid bass notes are just too... forward. But nitpicking aside, the bass line is very chill, very laid back and enjoyable. Still, the horns really steal the show, of all the wood winds, the HD 668B LOVES horns, the tone is perfect. Good warmth and body alongside with a nice bite. The high hats are amazing to listen to as well, they really feel like your walking on air, good shimmer and great extension.
•  Switching to the Nhoord Red V1
  1. That double bass line is a lot punchier on the Nhoord Red V1, it's not as mellow, though there is a much more balanced bass response. Still though, the horns aren't quite as bitey with the Nhoord either, it gives them a really beautiful sweetness! Overall, the Nhoord brings a more balanced response, which works nicely in this jazz piece! As I notice the paino more with the Nhoord than I did with the Superlux HD 668B.
• Switching to the HE 4
  1. ​​The HE 4 brings the best of both, the bass has just the right mix of punch and decay, the tone isn't uneven nor any part of it over emphasized. The HE 4 also brings a different sense of bite, lower softer notes are sweet, the more aggressive notes have bite to them! Where as the Nhoord gave everything the instrument played a sweetness, and the HD 668B made it all kinda of bitey, the HE 4 brings each quality more naturally. A sweet soft tone, then instantly a shift to a brighter bitey note, then back to a smoooth sweet tone. Gives you a very real sense of almost being right there with the band! An the high hats, are equally nice on all three. 

 
Igor Levit: Bach, Beethoven, Rzewski #01 Goldberg Variations, BWV 988 - Aria with 30 Variations 

• Superlux HD 668B ​
  1. SHOUTY, is really the only description I can give about the HD 668B with regards to this track. Being a piano solo, the uneven nature of the mids is PAINFULLY obvious. The upper mids are excruciatingly SHOUTY, in comparison to the over emphasis mid bass and recessed low to central mids. While I really do like the HD 668B with classical, it does not do well with... a singular piano... at all... it is just a cacophonous mess. That said how ever, amidst this mess there is some nice mirco detail! This song has the weird sound of... something moving around in the background. Almost sounds like some one dancing in a ball room as the music is being played... and that detail is present... even amidst the SCREAMING upper mids...
• Switching to the Nhoord Red V1
  1. Even the Nhoord Red V1, is still too warm in this track for my tastes. It's... forward mid bass is still annoying me. The mids are much more balanced with the Nhoord, and there's a real sense of beauty to each strike... but this beauty is short lived by a some what forward bass. Which can over shadow the beautifully delicate and short lived strikes, that our pianist makes such simply gorgeous use of, though... the  Nhoord Red V1 does not maintain the oddly distinct little bit of detail, that is our dancing background noise
• Switching to the HE 4
  1. ​​The tone is VERY proper, delicate when needed, you can hear the weight of each stroke, the heft of the deeper notes does not over shadow the beauty and delicate tactility of the quicker strikes, and amidst this astonishingly beautiful song is the gentle sound of a dancer moving around in the background. Where as the HD 668B can be a bit forceful with it's mirco detail, the HE 4 is much more polite and spacious with these details. Thus adding a very surreal sense of ambiance. 

 
Collected [Shadow Child] #07 The Verdict (Part 2) (Edit)

• Superlux HD 668B
  1. Fun, this track is just a lot of fun with the HD 668B. The slight v shaped frequency response really does add something FUN to this deep house track. While rolled off, the sub bass does still manage to add a little bit of texture. Still, the dominant upper mid bass does really steal the show. Great decay, nice sense of weight and slam. The overly dry bass of the HD 668B is a compliant for this track! My only gripe is the vocals are a bit thin and un-natural, other wise the synth horns and percussion all layer in nicely.

•   Switching to the Nhoord Red V1
  1. ​​Sadly, the Nhoord is much more balanced, so while there's a more satisfying deep bass kick, the overall lack of bass in comparison to the HD 668B is kinda boring. Though, the vocals have a really beautiful naturalness to them, and the bass has a touch more texture but... and more atmosphere... but all of these extra qualities are the result of a... kinda boring bass line. When the bass is suddenly not THROWN IN YOUR FACE, you have time to appreciate the extra details. Like the beautiful decay of a metallic chime, or how the high synth horn echos on the left and right channels... those extra details are really cool... but not an equal exchange for a less fun bass line.
• Switching to the HE 4
  1. ​​Best of both worlds, for an open back the HE 4 has a VERY AGGRESSIVE, HEAVY weighty bass slam. With very little added decay. Not to mention it's almost linear from 20- 200 hrz, so the texture is almost un real, feels like a big heavy monster breathing on your neck at times, and all of this massive slam and weight along side the really cool details that I first noticed in the Nhoord. My ONLY problem with the HE 4 and EDM is that truth be told, it is a bright headphone, and this brightness is really only noticeable when your listening to any synthesized or digital sounds. So by the time, I've got the volume to right around 87-88 db's SPL the highs do get a little hot 

 
... And Justice for All - [Metallica] #05 The Shortest Straw

• Superlux HD 668B 
  1. The long last bass line makes it's return! The very in your face mirco detail in the HD 668B works wonders for the very buried bass line in this classic Metallica song. While I loooove getting to hear that, there's a real lack of bite. The guitar riffs have a nice crunch, the double drum has excellent attack, but the song as a whole lacks bite. An while turning up the volume does "technically" help, you still left with an over zealous top end... and not enough bite.
• Switching to the Nhoord Red V1
  1. ​​The Nhoord Red V1, brings back the bite! The vocals have a nicer more tactile edge to them, and the drums have a better defined space within the sound stage. Though... while the sound stage is better defined it's not quite as big as on the HD 668B, the Nhoord trades width for definition and intimacy. The bass line is still audible, but it's a bit over shadowed by a nice heavy bite on the guitars! 
• Switching to the HE 4
  1. ​​Best of both Worlds again, there's all the bite you could, want alongside a crazy amount of impact in the double bass drums. All without overshadowing that bass line or sounding too "intimate" 

Following the HD600s, I had simply obtained the Superlux 668b as a point of comparison and reference, as it is a well know and popular budget Headphone. My brother also uses a pair that he swears by (unless he is borrowing my SHP9500). The simple truth is that the Superlux 668b cuts through detail like a knife. Rough and tumble sound, like a cowboy; he gets the job done well with only 6 bullets, or 50 bucks, then tips his hat while he walks off into the sunset. Superlux went full high-noon and checked off all the boxes for a compelling sound without robbing the evening coach. The Superlux 668b follows suit of the KSC75 in being an incredible value in sound quality. Better still, the Superlux has some very well though out design features.
 
The tiny little 3.5mm cable (i dont know what else to call it) that is used to attach various lengths of auxiliary extension cable is fantastic.
As I am sure a few of you thought to do....

 
or better still.....
 

 
To be honest, this could perhaps be used with a bluetooth adapter (though you would look goofy to say the least.)
 
The sound signature strikes me as V- shaped with treble emphasis. I find this quite nice as a contrast to my Sennheisers and Philips. My warning label for this headphone is that they can be harsh. They can be very harsh. This also adds an extra sense of clarity at times. Going from the 668b to HD558s is like going to bight sun to a damp cave. 
I find the sound of the Superlux 668b good enough that I would consider mixing with them.
 
As I have found with pretty much every headphone (except IEMs) more clean power adds to the extension and balance. The Bass got some more growl from the Project Polaris as well.
 
The soundstage is there, but often times I felt as if they were trying to be a tad closed by shoving treble into my skull, while others they seemed almost distant. I have not pinned down exactly what causes this, but it is likely to do with signature. 
 
The fit is another point on which the Superlux 668b can go wither way. The stock pads are quite large and there is foam inside (not big enough for my dumbo ears though.) The wing things on top are meh at best, and honestly I would prefer a real headband, something like the AKG style that the Samson SR850 has. The good news is that the wire frame can simply be stretched or bent to whatever headshape you want. Tight or loose is all up to you. I only loosened mine a bit, but kept it tight enough that I could try to muster out some more bass.
 
The pads are fine. Honestly sub-par compared to many other headphones, but for the price they are fine. I got some velours for $8 on amazon. I do not like them as they were a little smaller than the stock pads in inner height, so I went back to the stock pads. I also noticed that the velours leaked more bass, but also seemed to add a little to the mids.
 
Song recommendation: Gunslinger – Avenged Sevenfold (fits the cowboy theme)
I quite like what these to with metal in particular.
 
Comparisons will be here:
 
Superlux 668b vs HD600:
 
The Superlux is much less forgiving and harsh, more treble, and gives excellent detail. The Superlux strikes me as a bit V shaped with emphasis on the treble comparatively. Strangely enough, I find the Superlux to be more forward, yet somehow less intimate. The 668b is definitely leaner than the HD600. Some people will likely find the HD600 to be relatively veiled in comparison. Some frequencies are a bit under-represented on the Superlux while the HD600 is more neutral. The real difference for me is that it sounds like you are monitoring a recording on the Superlux, While listening to music on the Senneheiser.
 
Superlux 668b vs HD700
 
Wile the Superlux can sound more inside your head at times; the HD700 is very 3 dimensional. The HD700 also carries more warmth and has more body. They both have similar treble spikes as well. I also get less splash from the HD700, as well as more resolve. I find the bass to roughly the same in texture, and tightness. The 700 again carries a level of warmth in the bass, which can make the 668b bass seem better to those who want raw visceral bass. The Superlux can be sibilant sometimes, while this rarely happens with the HD700. I also find the HD700 to be smoother (minus the treble spike) in typical Sennheiser fashion. Price/performance: Superlux wins by a landslide.
 
Superlux 668b vs Koss KSC75
 
These two sounded surprisingly similar. The Superlux is more detailed for sure, as the KSC75 gets more bass texture. The price increase over the Koss is clearly all going to the build. This shows as I am not afraid to be a little rough with the 668B. I also found the sound of the 668b to be more analytical than the KSC. In fact, I would even mix with the Superlux.
 
Superlux 668b vs SHP9500
 
Well. This is where the rubber meets the road. The SHP9500 is much more natural, smoother, and, to me, far more comfortable. The Superlux fights back by having more aggressiveness and is undoubtedly more crisp. The SHP9500 seems a relaxed compared to the 668b and The SHP9500 won in the intimacy battle.
 
Superlux 668b vs HD558
 
I found the Superlux to be far cleaner, and far less mid-bass heavy. The treble more rolled off with HD558. They are in many ways opposites. I think the Superlux is the more capable headphone, but I still find the HD558 to have that special something many headphones simply do not. The Superlux is a much better value for money, but it would not surprise me if more people prefer the HD558.
 
Conclusion: Like many, I have been in the tight budget category, and many see no reason to spend more on headphones. For those people, this should be a banner to fly. The price/performance is unquestioned in my mind. This is a genuine steal at the asking price. I will recommend the Superlux 668b without question.

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The product was provided to me free of charge by Gearbest (thank you @George-gearbest) for the review purpose in exchange for my honest opinion.
Manufacturer website: Superlux, Available from Gearbest.
* click on images to expand.

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Intro.
 
Even so I have been focusing lately on testing and reviewing flagship IEMs and CIEMs, majority of which cost north of $1k, in this write-up I’m going back to my roots and will talk about sone of the cheapest pairs of headphones I’ve tested to date.  Keep in mind these are full size semi-open cans, not another budget pair of in-ears or earbuds where I have seen even lower prices ($5 Monk being the prime example).  But still, at $35 or sometime even down to $30, I haven’t heard full size headphones that cheap with a sound that good!  Don’t mistake this level of excitement for “OMG, this kills any $500-$1k pair of headphones”, let’s be realistic.  But I have no problem saying these $30-$35 headphones have a build quality of other $60-$70 pairs and sound quality that can go head-to-head with a number of full size cans up to $200.
 
3+ years ago when I started writing my audio gear reviews, my focus was solely on budget earphones, trying to find the next Giant Killer.  But you can’t judge something to be a “giant killer” without experience of testing more expensive mid-tier and flagship “giants”.  Now, I made a full circle which gives me a more objective point of reference for comparison.  I also made an interesting observation about this HD668B model while reading various impressions since its release in 2011.  Many of the negative comments I read about it are no longer applicable to this 2016 version, which makes me wonder if Superlux already implemented some of the fixes and improvements based on the received feedback.
 
The ironic part of this write-up is that I wasn’t even planning to review these headphones and was asked to take a listen to them in addition to something else I requested to review from Gearbest.  I probably would have dismissed them just on the basis of how much they cost, expecting the usual of “you get what you paid for”.  But I was proven wrong, and now would like to share with you about my experience with Superlux HD668B after spending the last few weeks testing them.
 
Unboxing and accessories.
 
Unboxing experience of HD668B didn’t feel at all like I was dealing with a cheap pair of budget headphones.  A sturdy cardboard box with a nice cover image, a very detailed spec and a description of the design, and a complete list of accessories with corresponding pictures – the packaging box exterior already carried a message of the product being too good to be true for $35 price tag.  But as you know, the writing on the box could be just a part of the overhyped marketing, so without further hesitation I proceeded to lift the cover.
 
 
 

 
I was greeted with a set of full size cans neatly packed inside of a cardboard insert and other accessories around it.  With everything out, I was looking at a generous size soft nylon carrying pouch/bag, two extension cables, 6.3mm adapter, a peculiar cable clip, and a printed manual/guide.  The nylon pouch felt neat and durable, branded with Superlux name.  I consider adapter to be a filler, but it could come handy with full size headphones when used in a desktop setup.  The clip made sense once I attached the cable to HD668B, realizing it does a great job securing the joint if you pull on the wires.
 
 
 
 
 
Regarding the cable, it was actually an extension cord.  First of all, the whole idea of a removable cable in $35 pair of full size headphones is impressive.  But it’s not a traditional cable with 3.5mm male plug on each side.  To keep HD668B earcups slim, Superlux brought out the connector to the outside with a male 3.5mm plug and a very short extension cable.  As a result, you attach the provided “extension” cable to this external connector.  Included was 1m short cable, perfect for a portable use, and 3m long cable for other applications. You can also combine them together to form 4m extra long cable to give you more traveling room around the desk.
 
This "extension" cable idea was quite unique, though it would not be easy to find an extension cable with in-line remote if you need to.  But you can also argue that majority of people would prefer not to carry a phone conversation with these on.  With connector placed on outside, the earcup housing was definitely slimmer.  Also, the attached earcup connector cable is very short with a heavy duty shielding and impressive strain relief.  Not 100% sure if the cable clip was really necessary since the mating of the cable was already secure, but it never hurts to add another level of protection.
 
 
 
Design & build.
 
The first look at HD668B brought back the memory of my recent review of ATH-A2000Z with a similar wire-headband design and 3D-wings.  Honestly, I was expecting a flimsy plastic construction, but surprisingly found it to be not bad at all.  Starting with a headband, you have minimalistic design with a support of 2 steel spring-wires, assuming one carries a cable going to the connector on the left side.  I have an average size head, and it was a bit tight at first but loosened up after a short time of being stretched over the soccer ball.
 
There is no height adjustment like in traditional full size headphones, and instead you have soft foam “wings” on a spring metal wire which self-adjusts as you put headphones on.  This could be hit-or-miss with some people who have big head or curly hair (the last comment actually came from my wife).  For me personally, it works quite well since I’m already accustomed to ATH-R70x and ATH-A2000Z, both of which have a similar “wings” headband support.
 

HD668B next to ATH-A2000Z
 
Though the headband spring-wire felt solid, these wings felt a bit flimsy in comparison, especially a thinner wire attachment - just have to be more careful when taking these headphones off you head.  Closer to earcups, the headband wires are held together with a plastic joint piece on each side, kind of reminding me of “y-splitter”.  These pieces look to be replaceable, if needed, and made of a solid plastic material.  I found HD668B to be relatively comfortable to wear, and with evenly distributed 220g of weight it felt feather light.
 
Moving on to earcups, here Superlux took a page right out of AKG240.  I have no doubt that Superlux engineers were inspired by AKG design, even bringing the connector to outside, though keeping a standard 3.5mm TRS plug for “extension” cable attachment.  The back of earcups also has a similar to AKG circular vents, intended for a semi-open back performance.  The inner middle part of earcups does pivots to adjust around your head, and it works quite well.  The pleather earpads are easy to remove and to replace, especially if you want something softer.  Since the earpads look universal and stretch over the top of earcups, I’m sure replacement alternatives won’t be hard to find.
 
The included original pleather pads are OK, but they do get a little hot after extended use.  It definitely makes sense to experiment with different earpads to hear how they affect the sound.  Just like eartips of in-ear monitors, you can fine tune the level of low end extension and impact as well as being able to control the top end.  In addition, once you remove the earpads, you can also experiment with different dampening foam materials to figure out how it affects the sound.  Here you will find plenty of room for modding experiments.
 
 
 
 
 
 
 

 
The fit.
 

 
Sound analysis.
 
Before jumping into sound analysis, I left HD668B on burn in for about 100hrs, making sure their dynamic drivers are properly conditioned.  Honestly, I didn’t have too high expectations for these, still being a bit skeptical due to the low price.  As a result, my surprise was more shocking when I discovered HD668B to be tuned with a tasteful V-shaped full body sound signature and warm detailed tonality.
 
In more details, I hear a deep low end impact, down to a textured sub-bass rumble and a tight mid-bass which has a bit of a hollow punch.  The bass is rather well controlled, articulate, and there is no spillage into lower mids.  The lower mids have a nice full body, but there is no muddiness or congestion, while upper mids are clear and detailed with an organic presentation of vocals - not too much upfront, just pushed a little back due to an elevated low end.  Treble is surprisingly crisp and detailed, with a good extension, and some nice airiness which helps with layering and separation of instruments and vocals and contributes to higher resolution of the sound.  If I have to nitpick, perhaps upper frequencies have a little shade of metallic sheen, but it wasn’t that bad especially when you pair up with different sources.
 
Soundstage has a good expansion in width, not super wide but definitely way above the average.  The depth is not too far out, but you still feel like a few rows in front of the stage, giving a soundstage perception of spreading left/right around you without going too deep.  Layering and separation was surprisingly good due to well controlled low end which keeps the sound clean from muddiness without spilling into mids and extra sparkle in treble which adds some air between the layers of the sound.
 
If you are looking for a good isolation, these are definitely not the best for listening on the bus or in the library, after all - this is a semi-open design.  But it wasn’t leaking as bad when you step away 2-3ft.  I’m sure some will find this semi-open design even beneficial since it could raise the surround awareness without losing too much of the sound details.
 
Sound comparison.
 
Considering 98dB sensitivity and 56 ohm impedance, I found HD668B to be a little less efficient in comparison to my other headphones, and as a result I had to push the volume a little higher.  Also, it’s clear that more expensive headphones have an edge in build quality and material selection.
 
HD668B vs ATH-MSR7 - HD soundstage is wider, while MSR7 has more depth.  HD has a deeper sub-bass and a little stronger mid-bass, while MSR7 has a more balanced low end.  I also noticed that HD has a little faster attack of the mid-bass while MSR7 is a tad slower.  Lower mids are similar, while MSR7 upper mids are more forward, balanced, and a little brighter in comparison to HD.  I hear treble as being similarly crisp and well defined, though MSR7 has a little more airiness and a better extension.  Overall, the biggest difference here is a more balanced signature of MSR7 versus lifted low end and slightly recessed mids of HD.
 
HD668B vs ATH-M50x - HD soundstage is wider, while M50x has more depth.  HD has a deeper sub-bass while mid-bass is similar, perhaps with HD having a bit stronger impact.  Mid-bass speed is also similar.  Lower mids are very similar, while upper mids in M50x are more balanced and more forward in comparison.  Also, M50x upper mids are a little smoother and a touch warmer.  HD treble is crispier and has more airiness in comparison to a smoother M50x treble.  While I enjoy M50x comfortable fit for extended listening on my laptop, here I actually preferred the sound of HD.
 
HD668B vs SoundMagic HP150 - HP150 soundstage comes closer to HD width, but not quite there, and HP150 has a little more depth.  Also, HP150 sub-bass reaches deeper to the same level as HD, but a little shy of HD quantity, and mid-bass in HP150 is a bit lower in quantity as well.  HP150 lower mids are leaner and upper mids are a little more upfront and brighter with an advantage of slightly improved retrieval of details.  Treble is rather similar, being crisp, bright, snappy, and with enough extension and airiness to make the sound more resolving and layered.  Actually, after closer listening I hear HP150 being a little brighter in upper mids/treble.
 
HD668B vs Oppo PM3 - HD has more width and both have a similar depth.  HD has a deeper sub-bass extension and faster mid-bass punch with more impact.  PM3 mids sound more congested in comparison to HD, though they are more forward in presentation.  Actually HD upper mids are brighter and more detailed and the treble is crisper, better defined, and with more airiness.  PM3 has a very smooth laid back sound while HD has a faster punchier brighter and more detailed sound.
 
 

 
Pair up.
 
Note 4 – Pretty good pair up, though have to push volume closer to the max.  The sound becomes more balanced with a little less impact in mid-bass and a softer sub-bass.  But mids are still detailed and treble still has a great extension.  I also found upper mids/treble to sound a little brighter.  Good soundstage expansion.
 
LPG – here you will find a great transparent sound, deep extended sub-bass, punchy mid-bass, clear and detailed mids, crisp and airy treble.  Soundstage had an impressive width.  It was a great pair up, and I enjoyed the sound quite a bit, definitely giving HD668B giant killer stamp of approval while driving this pair of $35 headphones from this $2k source.
 
DX80 – I found a surprisingly balanced sound with mids a little more forward instead of pushed back, but I think this balanced sound sig change was due to sub-bass and mid-bass being scaled back a bit.  Very clear and detailed sound, upper mids are a bit thinner and vocals were not as organic anymore, treble is crisp, clear, and airy.  Soundstage was wide.
 
X7 w/AM2 – in this pair up I found sub-bass and mid-bass scaled down a bit, making the sound more balanced.  But it also felt like upper mids came a little more forward.  In addition, the bass was tight and punchy, clear detailed mids with plenty of body, and crisp airy treble.  Soundstage was wide.
 
L3 Pro – here the signature was back to v-shaped, with a good sub-bass extension and a nice rumble, and tight punchy mid-bass with a decent impact.  Lower mids have plenty of body, and uppers mids are clear and detailed but also a little harsh with a metallic texture and a touch of sibilance.  Treble is crisp, airy, extended.  Soundstage was wide.
 
AK120ii – in this pair up I found a little bit less sub-bass, still tight punchy mid-bass with a bit of a polite quantity, lower mids had natural full body and upper mids were a little thin, bright, and quite detailed. Treble is crisp, airy, extended.  Upper frequencies have a bit of a metallic sheen.  Soundstage expansion was pretty good.
 
Conclusion.
 
I’m starting to notice a pattern where many of my “giant killer” discoveries happen by accident, when I’m not even looking for another budget pair of headphones and instead just asked to try something new for fun.  Did I get spoiled by flagships?  It’s hard not to, and I’m being honest about it.  But sometime it’s refreshing to take a step back and to listen to these amazing gems that cost so little.  From $5 Monks to $10 KZs and now $35 HD668B – these are all amazing discoveries that show you how creative a manufacture can get by challenging themselves to squeeze every ounce of performance and sound tuning on such a small budget.  HD668B is a real deal Giant Killer, and either if you are on a budget or just want to humor yourself with another budget discovery – I have no problem recommending these semi-open full size headphones because they don’t just look cool and ready for modding, but they also sound great!