Hi all, 
 
Preface:
 
I was debating selling my HD600's because they seemed to have no low end straight out of my "pro" audio interfaces.  They lacked character, and did not sound better (they sounded inferior) then my modded grado sr60e's (bass vented, Amazonbasic pads=basshead grado).  I was skeptical about the possibility of adding an amp to a headphone I didn't like.  However, eventually succumbing to the notion I have never owned a "real" headphone amp, I bought this little dot mk ii from amazon for about $180 USD and a pair of Voshkod 6ZH1P-EV tubes from ebay (Yen Audio, They are great and prompt) for around $40-$50 USD for the pair.   I chose this amp (rightly so, now that I have it and use it may I add) and tube combo because I would have many configurations to work with / chances to obtain a good sound synergy with my HD600's.  I did not use the stock tubes at all however, because the voshkod's arrived before the amp   .  
 
Do you need one:  (read: you need one)
 
This amp demolished my idea I didn't need a "real" amp.   I am aware of the honeymoon phase and false judgements used to rectify one's purchases.  I'll put it this way:  any of my fairly mid-high end recording focusrite audio interfaces or mixer headphone outputs sound like airplane-supplied radio outputs when compared to the Mkii.  It is not subtle.  I also appreciate I have no understanding on why this is- according the output ratings on other gear I have, the response across the frequencies should not change this drastically based on amplification source, i.e.  the bass/low end shouldn't suddenly become epic on my hd600's while somehow still maintaining EQ-frequency-equilibrium...?  Anywho, this is evidently completely possible and demonstrable with electricity, case to point=this amp (and quite honestly I bet most other proper headphone amps prove this electrical fact too, I just have experienced it for the first time...) 
 
Random listening notes:
 
The amp makes cymbals "splash" in a nicer way (read: acutely noticeable and impressive way).  It is hard to describe, but when listening to metal and post hardcore recordings through tidal and a competent focusrite dac, the lack of grittiness on the cymbals is the most profound change (besides the walloping low end) 
 
Vince guaraldi, my favorite jazz pianist of all time, does great with the amp.  The double bass in the trio and group recordings of his really pop and thud with the mk ii.  And those jazz ride cymbals decay with just the right amount of... whatever they need to sound as full as they do now.  
 
Dirty loops, kimbra, singer songwriters, etc (the usual new stuff on the block) all benefit from the new low end I am hearing, but that timbre and decay really livens up recordings I don't usually call "spicy". 
 
I can critically listen (and maintain more or less focused attention) on albums/hours at a time, which is great.  What a way to relax and be intellectually stimulated at the same time!
 
MOD NOTES:
 
I tested my Voshkod tubes at all the gain settings.  please note the are a PITA the get to, (remove the bottom plate with the front and back plate screws- the whole bottom will come off with the feet still attached).
 
with a setting of 1 on both switches, the sound is perfect.  Nuff said.  No background noise, the beefy extended response I wanted. 
with all switches off = high gain = bad internal buzz problem (not my power, I have an excellent power conditioner)
With a setting of 2 on both sides: sounded like the mids were raspy, I have got no idea why.
WIth a setting of 1+2 on both sides: sounded weaker than just a setting of 1, like a cup of joe with too much milk  (I digress, I'm running out of audiophile likenesses)
 
The amp gets REALLY hot (what do you expect, you have 4 while-hot burning things on your desk)
 
let the tubes play for a while before making conclusions, but I mostly find they just need to warm up for a few minutes before a listening session (like, the sound is unintelligible until the tubes hit 88mph and go into warp speed mode, or something.) 
 
ALL IN ALL:
 
I am now a believer in good amplification.  The timbre, decay, and bassy-goodness I can now hear while keeping a relatively flat response (to my ears) makes HD-600's truly worth it.  The sound is non-invasive but lets the listener hear as much of what they turn their attention to- just like a concert at an intimate venue.  
 
 
Peace, Jay

These are so weird.  
 
I snagged them on a whim, wanting the grado flavor for cheap after having listened to some pair of grados at the hifi store a few months ago.  the sound stayed with me after all this time;  yet technically they are "terrible", i.e. no bass extension, strange supra aural fit, leak sound everywhere but are easy to drive (plugs into phone, but not for the subway....ever....)  have a sibilance that rules out sounds like krewella and other brightly-mixed albums.   everything I say so far points to a pair of really useless cans.  YET
 
I own a lot of headphones.  I like the different uses and sounds they each impart, from ones for the plane to ones really intended for mixing and recording/monitoring.  
 
I am a musician.  I've been teaching music to kids this summer, and one thing we do a lot of in the studio with the kids is practice "critically listening" to tunes and really learn to pick them apart in an intellectual, musician-ly way.  Ironically, despite being smitten with music, I am pretty mediocre IMO at critically listening to music alone.   I often just sort of let it wash over me (which is fine but a terrible application for grados like these SR60e's)  I will never forget how excited these kids would get over hearing new things in the music the sort of liked, and having that "ah - ha" moment about how bits of the arrangements fit together to influence them.   
 
All that amounts to:  I remember the revealing sound of grados, so I buy these.   Instantly feel like they suck.   Then, more  than an hour later, I realize I am still wearing them, straight up devouring vince guaraldi hits like "blue charlie brown" etc.   I felt so touched by the liveliness of the music, with every phrase seeming to be placed straight into my lap for me to engage with. 
 
I found royal blood's "better strangers" and "loose change" seemed to become practically animated, so epic and alive.    
 
These headphones fill a very small but very enjoyable niche in headphone land.   When you want to focus ONLY on the music, but only have a phone, and are in a space open cans would be non invasive, and already own ideally better equipment for long sessions and more serious bass extension, and feel you may be a music nerd.....   then these are your pick, they are really inexpensive for a whole new flavor.  
 
Thanks!
 
Jay

I have listened to many headphones, almost exclusively for monitoring and mixing applications. Mostly living in sennheiser land, I have owned/own hd 280, hd 598, and hd 600. I have listened extensively to the HD 800/800s, as well as audeze EL-8O, and sine. I am also very familiar with the Bose in-ears and QC range, FWIW. I wanted a closed headphone for public transit, one I wouldn't feel bad about destroying by accident. Well, I had a $25 amazon-brand discount after a slight billing snafu with Amazon, so I figured, $15 headphones, completely free to me, why not?

Firstly, these headphones have -really- opened up new genres to me, straying so wildly from my jazz collection in Tidal into Paramore, then into screamo-punk Escape the Fate. This is due to the simple fact I have never really heard a "fun" pair of supra aurals, basing my entire experience in a world where "EQ is dead flat" and dynamic over-ears reign supreme. The driver sits close to the ear, making metal and rock really "tight" and "punchy". This also lends itself to the bass proximity effect, making lows seem fatter. This is the opposit from my hd 598, where I felt the bassier frequencies dissipated before they got to my ear.

With super, uber bassy songs, one can literally feel the whole ear piece wobbling and vibrating against your head. A novel experience to say the least. Amazingly, I found a fairly low amount of distortion at really loud volumes (compared to low volumes: yes, these $15 cans distort a bit), so head bangers and EDM-heads can rejoice.

These are super efficient, and get at least as loud as apple ear buds with an iPhone when I A/B-ed for matched volumes.

Here is the kicker. I no longer listen to my $400 hd 600 for "fun" anymore, except for long jazz or critical thinking/listening sessions. I can't wander around the house in my own sonic bubble with the 600s, and I DO NOT LIKE metal or rock with my hd 600's.

Cons? Because of the V signature, the mids are very recessed, and will make some vocal performances seem less intimate; that being said, I, inversely from the 600's, don't care to enjoy intimate performances and sweet venues with young singer/songwriters when I am wandering around doing stuff. That is for evening sits with a bigger rig. I want exciting sound when I am doing things at home or on the go, and these fit the bill. I regularly have been taking these over Bose sound sport, my other wander about set. Now those only do sweaty tasks, because I simply am getting more out of my music with these funky, blue, plastic headphones.
Thanks, Jay