Preface: Comparisons (click here to read).
So, I already reviewed the little mini “FX Audio Tube-03” (years ago). That was my only “tube thing” then. I also got a Syba Audio tube from a friend a few months ago (in exchange for other audio equipment I sold him).
About the Tube-01: It was only a tube “pre” (RCA in, RCA out, that is all). Without EQ, the difference it made on the sound was subtle, even when tube swapping to Voskhod 6J1P-EV tubes. The EQ was fun too. But the non-standard output (the RCA output was more than 2V) made it hard to pinpoint where the volume level should be, same for the EQs having no “notch” where “EQ OFF” should be, and distortion often happened, especially when listening to electronic music, while just trying to get the same amount of bass from non-tube amps. Still, it did good things with speakers, where distortion was less audible than with headphones. Still, distortion was always a problem, or all-around weirdness when listening to electronic music, so I ended up putting a 60$ Burson opamp I got for free (in this 40$ thing, yes). The result is a (definitely-not-neutral) abundance of mids and bass, and now, as Zeos says, “it smooth jazzifies everything”.
About the Syba Audio: It was more of the same. One tube, one EQ (with notches and an on/off switch this time), and easy-to-hear distortion with some headphones. A ton of inputs, outputs, and a decent ESS DAC included (I mean, decent for a ~200$ box full of functions). Still, another one that (without the EQ) “smooth jazzified everything”. i.e.: Added a bit of bass, “smoothening” the overall sound.
So, why did I just gave both of these to my friends, and then bought a (used) Little Dot Mk2 instead? And why do I not regret it one bit? Because I wanted to know what a real tube amp was. A real tube amp being, tubes, but also transformers (…the big box behind the tubes), and above all, for headphones, OTL technology. And now I know that OTL definitely does things different, and is definitely a step above.
Little Dot Mk2: Your entry point into life-like voices, soundstage and 3D audio.
Infos: First, from now on I’ll just call it the “Mk2”. And for the DAC, I used the Xduoo XD-05 Basic (there’s a 3.5mm-to-RCA line out, it’s my desktop DAC here). This is an A/B comparison between the Xduoo XD-05 Basic headphone out (it never turns off, very useful for amp A/B tests), and the Mk2’s headphone out. For headphones, I used Nad HP50’s and Sennheiser HD6XX’s (and Sony MH755 IEMs). For the tubes, I used the 6J1P-EV’s (I also have GE JAN 5654W’s). The Mk2’s dip switches were set to “gain level 5”, see below in “Bonus #1: Gain levels” (hidden dip switches!). More comparisons below in “Bonus #2: The RCA out”.
Yes, I have Sennheiser HD6XX’s. Yes, it adds much needed bass to them, which is nice. But praising OTL tubes “only for the added bass” and “only for high-impedance headphones” is a huge mistake. Because what you get with this Mk2, whatever the headphones you use, is a great sense of depth and width and a ton of added dynamics. My 32 ohms Nad HP50’s also sound great, if not better than my 300 ohms 6XX’s, with it. Hell, it even makes my 10$ Sony MH755’s sound like endgame stuff (these are IEMs… the big volume knob was at like 10/100). The Mk2 just appears to do great, great things to audio, whatever the headphones (or even IEMs) you use, whatever the impedance… As long as it’s not planars: Planars are basically engineered to work exactly the opposite way, so my Fostex T50RP’s don’t like it. If your planars work well with your OTL headphone amp, you’re lucky, it’s an exception.
In short, whatever the headphones (or IEMS!) you use, and whatever the impedance they are, the Mk2 adds a ton of life and realism to voices, and a sense of scale, to your DAC, separating and giving every instrument its place. And as far as I know right now (and that’s the most important part) it does all that without removing anything. You will still hear your DAC and all the details it gives you. The frequency range (bass, mids, treble) will be basically unchanged (with 6J1P-EV tubes, for me at least), and you will still be able to blast electronic music without hearing any distortion, and bass will punch as much as with a non-tube amp. Everything (and I mean everything, from audiophile recordings to punk rock mp3s recorded like crap and youtube videos and vlogs) will just be… “in 3D instead of 2D”.
By buying a tube amp, I was expecting something to enjoy sometimes, and to switch to a “typical” headphone amplifier most of the time. But the opposite is happening. I was not expecting that. I view the Little Dot Mk2 as an incredible add-on to any DAC you like and believe sound great, but… “flat”. Which are most, if not all, “budget” desktop DACs. As in, possibly very detailed, but lacking width and depth and, sometimes, “soul”. If you like your DAC. You know it doesn’t distort. You know everything is there. You know all the detail is there. But you don’t “feel like you’re there”. And you need a mini winter heater. Well, this is for you. This Mk2, paired with a detailed DAC, is the most inexpensive way to feel like your room is the studio. I honestly believe I could mix and master music with this and not worry about any unexpected result while playing it back with anything else. And that’s why the Little Dot Mk2 is now my main headphone amplifier.
What can I add. Every time I believe the tubes do some “weird things”, I switch to the XD-05’s headphone out and realize it’s in the song. That’s… one of the downsides of trying odd and/or cheap tube things before getting this one, I guess: You become “paranoid”. But with a Mk2 and good 6J1P-EV tubes, you don’t need to worry. This is “tubes done right”. If you’re tired of hearing the entire band “inside your head” when you use headphones, this is for you (and you’ll get way more than that).
Bonus #1: Gain levels, the hidden dip switches (click here to read).
Again, all of the above is a review of the Mk2 with the gain set to level 5. You can set the gain with “dip switches” hidden inside the Mk2. Even with the dip switches set to the lowest level, (what I believe is) the standard, “gain 3”, my ears can hurt at 20/100 volume. So, why would you set it differently? Because with all my headphones, either with 6J1P-EV or JAN 5654W tubes, “gain 3” added treble and “air” to the music, which I did not want. Again, if you like everything from your DAC “except it sounds flat”, my recommendation is to leave it to “gain 5”. Being the middle ground it will also be a good all-around choice, whatever the headphones or IEMs you plug in.
What about gain level 10? Well, that’s another can of worms, because that’s where tube-swapping will make the biggest difference. 6J1P-EV tubes will boost the bass and mids, but will mute the “air”, even significantly lowering the treble in female voices (and whispers). Yup, oddly enough, it will “dull” the sound, and even diminish the soundstage – doing the exact opposite of why I love this amp. JAN 5654W tubes will not murder the treble, but I feel like everything still sounds like… molasses. Exactly like my Aune X7S (Class A headphone amplifier) with three gain settings, the highest gain level of the Mk2 is “11/10”, i.e.: overblown, overdone, muting the attack and speed of music, over-tubing, over-smoothening everything. I mean, I’m not complaining, it’s fun to have the choice. But “gain level 11”, both in the Mk2 or my X7S, is the “I don’t want to hear details, I don’t want to hear treble, I just want to sit back and relax” setting.
And no, my T50RP’s (planars) did not do better with higher gain either (still a bit wonky, even with the gain at 10, I hear sibilance and something weird at about ~200hz, for example). You know something’s wrong when even T50RP’s sound smooth and relaxing… these are my mech-looking, hard-hitting, analytical-sounding headphones.
But you forgot level 4? Well, my guess is it’s the same as level 5. Same specs, half the switches are affected, so half the resistors (or whatever) are in use. A quick google also mentions “Levels 3,5 or 10” on their websites, ignoring level 4. The Mk2 is made in China and “4” sounds exactly like “death” in Chinese, but I like to live dangerously, so it’s at level 4 now, and my headphones are not dead. 
(Honestly I opened and closed the Mk2 way too much and I’m tired and I don’t want to open it again to compare. Level 4 or 5 + Voskhod 6J1P-EV tubes, it sounds glorious either way.)
...and how to open the amp and set the dip switches? (Click here to read.)
You only need to remove the two lower screws from both the bottom of the front plate and the back plate to free the bottom plate (removing the four rubber bottom feet is useless – it’s only connected to the bottom plate). You don’t even need to remove the tubes from the amp, actually (if you like to live dangerously), because the dip switches are “under” the Mk2.
At first, I followed these instructions.
…and I had a surprise. Setting my Mk2 to “Gain 5” gave me high gain on the left channel and low gain for the right channel. Oops. This “Little Dot Mk2 guide” PDF (screenshot below) says “please remember to configure the switches on each red box identically with respect to the other”.
Yes, this is 2 switches for the 2 channels. Gain 5 is achieved by setting “1” to “on” and “2” to “off” on both switches. Don’t forget that I bought my Mk2 used. Maybe there’s been a few revisions (since 2010!), and the dip switches now work differently. But for mine, it’s one red box for each channel.
Bonus #2: The RCA out.
It’s not just a headphone amplifier: There’s also a pre-out. Sound goes through the tubes (and the dip switches!) before the RCA out. So this can act as a preamplifier for those stupid speaker amplifiers with no volume knobs (sorry). But also allows you to plug a non-OTL headphone amplifier and “tube your planars” (set the Mk2’s volume to 60-70, forget it, RCA out to an headphone amplifier and control the volume there).
Be aware that you won’t get all of the benefits of the OTL headphone amp from the RCA out (obviously). Still, maybe 50% of it. If you, for some odd reason, only intend to use this as a pre-out, be aware 50% of its “magic” will be unknown to you. But… that’s pretty much your only choice if you want to “tube” planars.
Okay, that was a long text. 100% worth it, though. He says, listening to his Little Dot Mk2. 
Enjoy!