Pros: Beefy sound, extended low end, amazingly obvious sound improvement over stock interface headphone amps
Cons: PITA gain adjustment
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!
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.