So first of all, since all reviews are subjective, I will gonna state some stuff about me first before I go review the Product.

My Background:
I am a amateur bassist. I am currently doing live mixing at church. I am someone on a budget so don't expect reviews about High-End Gear.

My Music Preference:
Rock, Jazz, J-Pop, J-Rock, K-Pop, EDM, Heavy Metal, Acoustic Music

My Preferred Sound signature:
I prefer some sort of bass boost, but not up to the level where the music sounds muddy. I would rather listen to no bass than listen to muddy bass. But my ideal bass boost is the bass boost where the Bass Guitar is kind of Forward. I like the mids to be flat, not forward, and not Recessed.. I like some treble boost as well but just quite the few steps before becoming sibilant. I like Female vocals to be lushy and quite edgy, especially to the high pitched ones. I like male vocals to be just natural sounding. I like to hear soundstage that is not quite spacious, and also not quite intimate, but I would rather like to listen to the details mushed in my ears rather than listening to music with a sense of space on everything like speakers. I do not care to imaging that much but it is nice to have.

My Testing Devices:
Pioneer HTZ-424DVD(Home Theater System)
Audio Technica ATH-CLR100(In ear Monitor)
Philips SHP2000(Semi Open Headphones, w/Mods)
iCore WavePods(Closed On-Ear Headphones)

Playback Devices:
Dell Latitude E6400
Dell XPS L702x
Neo N81(in Both Android and Windows OS)

Music Sources:
Youtube
Spotify
CD Files ripped in wav format.

Player of Choice:
MediaMonkey using WASAPI Output, Stock Android Music Player, Youtube, Spotify
 
Intro:
I bought the Fiio K1 out of curiosity because of the reviews from Head-Fi, Z Reviews, and Youtube reviews(So technically, I am quite the Average Consumer, not quite the Audiophile yet, but is starting to enter to the world of such). I also bought it because I am curious on having a DAC/Amp. As an Average consumer, I quite find the Device to be worth the purchase. Even if this is my First DAC/Amp, and my Only DAC/Amp, I know the Fiio K1 because I always use the device.
 
Social Experiments as a Review:

I tested my Little Sisters using the the Track Let it Go-Demi Lovato using Spotify,and using iCore Wavepods and used it to compare my E6400's onboard audio against my Fiio K1. I experimented them with a Blind A/B Test. They kind of struggled which is which, but they have heard some subtle difference.
 
I tested my friend to do a blind A/B test as well, I forgot the Track that I used. He didn't noticed the Difference between teh E6400's onboard audio against my Fiio K1.
 
I experimented using the K1 as A DAC for the Speakers. I used my Dell Latitude E6400 plugged into the Fiio K1 plugged into Pioneer HTZ-424DVD via AUX connection. I played a random jazz music on youtube, and the audio sounds clean and the layering works well. The sound's cleanliness reminds me of Some Background Music played at Hotel rooms with their speaker systems. 
 
Personal Experiences as a Review:
In my personal experience, I noticed that my Fiio K1 can push my SHP2000 to the louder volume compared to the E6400's. And the Fiio K1 does improve the soundstage and imaging. My onboard audio sound feels like the sound is always from the headphone driver, especially to my Audio Technica IEMs.
 
Compared to the onboard audio of my friend's HP Laptop, the difference between the K1's output is night and day with the K1 having the Favor. So if you are thinking of replacing your onboard audio for an improvement for the cheap price, you know that this little K1 could be your sound card replacement.
 
One thing to note that the DAC/Amp could get hot when you use it on hot places. Once the Fiio K1 Gets hot, they make your headphones and earphones become Needles. The sound feels quite piercing to the ear when it gets hot(This will serve as a warning to those who are wanting to get it but they live in Hot Areas).

Also, a thing to note is that the USB port affects the Sound of the DAC/Amplifier. I tested it on one of my L702x's USB 2.0 Port. It makes me wonder what happened because when I plugged it there, and used my SHP2000s. The Bass becomes intense, like the air is slapping your ears and it hurts. I plugged it into my L702x's USB 3.0 Port and The bass of SHP2000 became normal, like what I usually listen to my E6400 Laptop+the Fiio K1. On my Neo N81(in both windows mode and android mode), The Fiio K1 retains the Sound Quality, but just a little bit lower in terms of Max Volume compared to my E6400 and l702x.
 
One thing to note also, since it is not battery powered, like the other Portable DAC/Amps, it quite eats power from my Laptops to make it sustain its function. At least no charging required on this Portable DAC/Amp.
 
When I use my Audio Technica ATH-CLR100, together with the Fiio K1, I noticed that there is hissing on the IEM but the hissing is not that quite noticeable unless you focus on listening to the hiss, or you replugged the IEM back to the K1(This will serve as a warning to those who are conscious about hissing). I tried to max the volume of the K1, and luckily, the Hissing isn't amplified. Maybe it is more on the IEM itself
 
I tried my Android Lollipop Phone and it can't detect the DAC/Amp, so for short, it might not be working on all Android Devices.
 

Conclusion:
Despite the flaws that I pointed at specific scenarios, I still find my Fiio K1 to be a good Portable DAC/Amp. Solid build, Solid sound, Cheap Price. This will be the Average Consumer's Endgame DAC/Amp(Unless they want to enter to the world of much Higher Fidelity Audio)

I have purchased the Monk plus because of
a.) The hype
b.) Nostalgia on earbuds.

Note: I am reviewing the Espresso Edition monks

Build:
I respect the build of these CHi-Fi(Cheap HiFi or Chinese HiFi, pick your poison) earbuds. The cable is not so much tangly, but they still know how to do kinks. The Earbud shells feels like they are good enough as far as the build(Except being stepped on, the shells are still plastic). The weakest link would be the earbud foams which may get holey over time, and my Symmetry OCD makes me bother on the holey earbud foams, making me resort to buy earbud foams. No regrets on that though because they changed tonality.

Sound:
To be honest, I understand why these earbuds are being hyped. They have a good frequency response and they have a nice Transient Response as well. Soundstage can be sometimes wide on some recordings, Like real wide for such an earbud. Imaging is ok, but they are a little vague on the imaging. No frequency bleeds over the other. and the vocals are well presented well with a touch of airiness. On my case, the subbass is heard but not felt, which is a turn-off to some bassheads, but for me, it just does fine. The bass still has punch, but sometimes it also just sounds artificial. One thing is for sure, the people will respect the sound of these.

At one time, my parents are quite the suspective on my audio gear purchases, like my FiiO K1 USB DAC/Amp and AudioQuest Jitterbug, and recently I received these monk plus, they are quite curious, they tested the monks on my phone, and they have been enlightened on why I am willing to spend cash on Audio Gear because these monks has shown the way (My father is still bothered on why I should be still be buying Amps, especially he found the Monks to be already sounding amazing on just a phone).

I modded my monks by replacing the earfoams from a japan store out of curiosity and it was worth it, The foamies are quite smaller than the monks, but they would fit just fine.

With the replacement earbud foams, The bass have a gentler presentation compared to the stock earbud foams. but it has now a more proper articulation where the bass guitar sound guitar sounds like bass guitar(I am a bassist so I know) and it doesn't bleed on the mids much. Mids become a little less forward with the replacement foam, though it still has that fullness.The treble is quite rolled off, where they become just borderline screechy, because the stock earbud foams would still screech on strident music. The replacement foam also affects the imaging for the better. This mod would sacrifice the sound area covered by the earbuds where the earbuds just sound like headphones. But I have a full sized headphone so it doesn't quite matter for me.

They do excel in Bossa, Jazz, Rock, Classical, Metal, and the best genre to play with this is live concert music. The soundstage is so wide for an earbud. You may have reservations for the monks if you listen to more modern stuff where you want the meaty, catchy sound and vibrations, go for IEMs.

Comfort:

Comfort wise, they are comfortable, though I feel the heavyness of the earbuds on my ears. The foams quite vary in comfort at some point, some are quite scratchy in my ears when i twist to fit the eartips so you really have some need to find the perfect foam(Good thing there are 4 foams per each color, and there are two colors, so a total of 8 foamies to find your best comfy foamy. P.S. I use foams in same color because Symmetry OCD won't allow).

Sound Isolation and Leakage:

The foam thickness affects the sound isolation of these earbuds, Using thick foams, and cranking the volume, you are well isolated enough. Sound leakage, well, this is a turn off if you watch "those videos" because it basically leaks sound(which helps in soundstage and imaging so if you are alone, and you want that positional audio in movies, especially in "those movies", why not).

Comparison:
If there is something I will compare against, I will compare it to my Audio-Technica ATH-CLR100(w/modded eartips because stock eartips are bad on those). It may be unfair to compare an Earbud to an IEM but here is the thing, They both have the ability to present most of the frequencies smoothly, though the ATH-CLR100 has quite the less forward midrange, fast decay of vocals and a more snappy presentation on the bass and a stronger subbass(because bass is good when really felt). The ATH-CLR100 is quite the more forgiving on bad recordings, but a good recording is quite better to be heard on the monks.
Soundstage is better on the monks, but the imaging is more clear with the ATH-CLR100.

Conclusion:
They are a good Starter HiFi or CHiFi (because cheap $5, and again, pick your poison on your definition of CHiFi) device. If you are the someone who is on Audio Quality yet scared on Spending lots. Start with these. These CHiFI things are scary in the audio industry because they have a good price to performance ratio that the market will surely become real competitive because the CHiFi industry (FiiO, HiFiMan, SMSL, Superlux and of course the maker of these Monk Plus, Venture Electronics) is moving forward to better build and better sound (I suddenly realized I am also hyping as well). To be honest, this has almost become my best Audio Device, that it competes with my ATH-CLR100 on who will become my daily driver. It is just one order from your local shop or online store, and you might find these your end game for portable audio, again, portable audio. The monks are good just as they are.

Subjective Side notes:
At last, something that is good sounding as is. I have modded my ATH-CLR100 and Superlux HD661, and for the very first time ever, an Audio Device where I don't have to do Modding(to correct the sound signature(because DSP Correction/Equalization is just applied on one source so I still find modding to be a better way to change the sound signature)).

^I quite take it back. I still mod them with replacement ear foams to have that balanced tonality. I still do find it needs some work on earbud foamies and it could be better.

Something about me before the review:

I started live mixing as of April 2017 every Sundays. Still quite the amateur but I am quite learning about live mixing
I am formerly a bassist.
I prefer fun-neutral sound signature. A some sort of V-shaped or M-Shaped Frequency response to help me hear the low-end and the high frequencies. I am quite the detailhead, but I would love at least little amounts of soundstage and imaging to at least have some panning details to be clear.

Build Quality

These headphones are like Toy-Grade build like this is like meant for kids handling as far as the feel in the hand. This could quite survive in the hands of the klutzy kids. They feel like premium Auldey Toys in the hand. I have Nothing to complain about the build so much that these Superlux Headphones could quite rival the build of Sony and Audio Technica headphones. The pad replacement is easy, just a twist and pull will remove the pads, so at least they are moddable. The 3.5mm Male jack on the headphones may be something that people hate, but at least for me this system is way better than the Audio Technica and Shure's Locking 2.5mm to 3.5mm because Headphone extension cable is easier to find at least.

Comfort

These headphones are light on the head, but they quite clamp hard. You need to box-stretch these cans before using these. Pads are quite stiff, so you may need replacement of pads if the comfort really grits you

Sound In-Depth Review

Before burn in:
I was just testing this out on a local store(and bought it the same day) using the phone. I was quite interested at my first impressions because they are quite detailed and they have soundstage and imaging that is decent for a closed set of cans. Quite comfortable to listen still, not the most sibilant stuff.

When I brought it home and plugged it into my FiiO K1 DAC/Amp, they showed their true unburned nature. They are Treble Cannons, with the recessive midrange and recessed bass. So, as a tip, DON'T USE THEM EARLY WITHOUT BURNING IN

After Burn in:
I burned the Headphone Drivers 8 Hours at first. Noticed the treble recession like it is Night and Day.
Quite Sibilant still, but still quite unlistenable to some Tracks like Ai Shinozaki's Waruineko(Where the headphone shows the unforgiving sibilance of her shrill voice). I let them burn in for another 8 Hours and they become the perfect neutral cans. I will review them as of how they are now after the 16 hour burn in.

Bass:
The bass is good. Not the fastest but at least the details of bass tones are heard clearly. Mid bass can be clearly heard but it can be quite a struggle at some levels especially when there are electric guitar leads that also come with the song. It can rumble once it gets to the sub bass, but sometimes it is quite teasing to the ears especially on EDMs. One thing is for sure. The HD661 has a clean bass, no Muddiness at all.

Mids:
Mids are slightly recessed after the burn in. but they are decently audible for the details to come up and quite accurate enough for my monitoring tasks

Treble:
This is one of the main reason you will either buy or avoid buying this unit(unless you are into modding). They are quite strident. This headphone is unforgiving to shrill female vocals. The Crash will Crash unto your ears. The sibilant sounds will surely pierce your ear drums. The Electric Guitar leads are intimate with this Headphone, so it gives rock, and heavy metal justice to their recordings.

Conclusion:
I purchased these for $39(They are the only unit left). They are quite the economic monitors. I would recommend these for starters in the live mixing. For musicality, I would also recommend it but just be warned of the Treble cannons, and some comfort issues. Acoustic Modding Work is quite interesting with these since they are quite modular on the pads but the insides has less room for modding work so for sure they cannot be the next T50RP in terms of modifiabilty. One thing is for sure, The Price to Performance Ratio is excellent for these monitoring cans.

I will have to deal with the pinkiness of these monitors, I really want to be lowkey with black, white or gray color(They are the only ones available, but the sound really grits my heart). But who knows, girls might find me sexy with this pinky

Notes:
Some modding tips.
https://www.head-fi.org/f/articles/superlux-hd661-anti-sibilance-and-comfort-mod.19634/

Foreword:
In the audiophile world, there is a lot of "Snake Oil". Buy this and it improves audio, buy this and blah blah blah blah blah blah. Some people will get disappointed because sometimes they pay for something that doesn't feel like the performance of the item is worth the price.

When using USB DAC/Amps, the Port MATTERS. I plugged my FiiO K1 to my Dell E6400 and I thought it would sound like that ALL THROUGHOUT. Plugged my FiiO K1 in my new Dell XPS L702x and the sound became different, the Sub-bass become more impactful. I plugged it into the USB 3.0 port, and the treble got recessed to where I expected the treble will get into.

Now, to the Review:
I bought the AudioQuest Jitterbug for about $40. Just about the same as my trusty FiiO K1 DAC/Amp.

I tested this Audioquest Jittterbug using my FiiO K1 with my both my Superlux HD661 and Audio Technica ATH-CLR100 In-ear Monitors as output devices.

Here are my Test Tracks:

Catching the Moment-Lisa:
This is the very first track I tested. FiiO K1 by itself, and with this track, the sound is just so thin and congested. I plugged Jitterbug and the FiiO K1 did Justice to the recording. The first thing I noticed is that the Electric guitar got the resolving thickness that the electric guitars got more Engaging. Without the jitterbug, the sound is still clean, but the resolution is not there, and it is real boring.

Waruineko-Ai Shinozaki:
With the Jitterbug, The snap for the song intro feels natural, Ai Shinozaki's Shrill Sibilant vocals sounds natural. The reverb sounds natural. The piano sounds the natural. The bass notes become more noticeable. The piercing highs with this track is unbearable when using the FiiO K1 by itself with the Superlux HD661s but when I use the Jitterbug, the sound is still quite listenable, but still, you still feel "the treble cannon" nature of the headphones on this track.

Deja Vu-Dave Rodgers:
Without the Jitterbug, this Eurobeat track sounds boring, flat, and congested. With the jitterbug, the Separation of Vocals and the Synth did shine that makes them engaging to listen into. The track sounds layered with the Jitterbug, and without the jitterbug, the track feels analytical, but not bright enough either.

Summary and Conclusion:
The sense of separation, and the sound becoming more natural and transparent is the reason why I am in favor of this Jitterbug. For me it became the essential device for further improving audio, especially when using USB DAC/Amps. Overall, It is worth the price. This device is no Snake Oil at all, and this device is worth the price. This may be an odd product because at the Average Consumer's point of view, you could have just spent money on a USB Extension hub. But hey, even if some things are impractical on this modern day, like Vacuum tubes and earspeaker systems like Stax, we audiophiles just don't care(minus the budget). But for this $40 product, to some it is a no-brainer, but for me who is a budget-fi that wants to ensure Price to Performance, I think this is a no-brainer as well.

I bought this because there is hype on these ZST. I mean, cheap enough(Again I am into chi-fi), with a BA.

No need to explain some ergonomical issues since they are explained into pros and cons. Now to evaluate sound signature with these ZST

Sound signature on Stock wire:
Bass: Bass is kind of felt more than heard, but the major gripe of these is most of the bass has less presence and impact, especially impact. You hear that bass sounds from the violet shell.
Mids: Mids are recessed but still quite articulate and smooth enough(you will hear dat ) that you will be fine(you paid for a cheap earphone anyways)
Treble:Treble is one that I don't like on stock wire. They are somewhat too crystalline and gritty to my ears that it makes me stop using them. They are quite agressive and sharp on the treble

Sound signature on KZ SPC V2 wire:
Bass: Bass is now more forward enough and the bass tones are now more coherent with the mids and treble.
Mids: Now they become more forward as well, dat recessed mids issue, is now gone, and you feel that BA and the DD on its best.
Treble: Treble is now more controlled, and rolled of compared to stock wire(but still extended to let you hear details on highs)

Still a good buy but won't recommend it if you will just buy it stock.