In the last year, I changed my approach to headphone setups.  At one point I had a desktop setup for the HD650 (Bifrost + Lyr), a mobile amp with various portable cans I've owned (various FiiO products), and a small, work setup (Modi+Magni stack) for my Shure SRH840.  What I realized is that a large portion of the time I was not using any of that stuff and i was often simply using my Grados or KSC75s through my iPhone because it's convenient and I could do chores around the house or walk the dog.  I was spending a majority of my time with my least expensive setups.  
 
So I decided to simplify based on a few rules:
 
1) Any headphones had to be drivable by an iPhone and sound very good straight out.
2) All gear had to be all-arounders, in that it sounded good with any gear I own, and all amps work well with all headphones.  No more of this "this amp is for this headphone" stuff.
3) No more big desktop stacks.
 
But I also didn't want to compromise on audio quality either.  I was unsure if this was possible, but I wanted to see how close I could get.  So, I first sold off all the gear that didn't fit that program.  HD650, all my Schiit gear, gone.  
 
Headphones were the next big decision.  First move was the Shure SRH840.  To me, that's the best closed headphone in the $200 range (yeah, better than the M50X).  Grado SR225e came next as my fun, open can.  Then I demo'd the HiFiMan HE400i and knew that would be the cornerstone primary headphone.  
 
The next challenge became the DAC/Amp.  While everything had to be drivable by the iPhone, I did want something that I could use to take them to the next level.  And that was surprisingly challenging.  
 
People on here often talk about "scaling well" and primarily what they mean is that the headphones are hard to drive, and thus require an expensive powerful amp to properly power.  In my experience, easy to drive headphones can actually be more challenging to properly power.  Lower impedance easy to drive headphones require a lot of current, rather than a lot of pure power.  You also need a low output impedance for dampening.  The 02 was one of the first amps to really "get" this.  But it was shrill to my ears with a lot of headphones.  The Schiit Magni also offered this, but again, I didn't want a stack.  
 
I had always been drawn to Grace units, but A) they were the size of a rack unit and B) they were $1500-$2000, which was more than I wanted to spend.  
 
Things started to come together when I saw the announcement for the m9XX.
 
The m9XX on paper looked about perfect.  
 
The ultra-low impedance meant that it would drive a lot of current (ohms law states that as the output impedance goes toward 0, current goes towards infinity), especially since it could deliver 1000mW at 32 ohms. With an output impedance of 0.08, this meant that the current it sent to headphones was literally near infinite.  
 
The design was spot on: simple, ergonomically appealing, attractive.  It had several features I found very appealing as well:
1) Crossfeed.  To me crossfeed is nearly required for long listening sessions to avoid fatigue.  Without it, it's like my brain gets tired trying to reconcile hearing sounds consistently only out of one ear (this almost never happens in real life, so it confuses your brain).  
2) linear decibel stepped volume control.  Most amps have a volume control where power levels are just some arbitrary points. often times halfway through the dial isn't half power, or half volume or anything in particular.  On the Grace m9XX, each spot represents an actual half decibel.  This makes it possible to fine tune adjust the volume on an IEM and a planar magnetic headphone without using a gain switch (and thus changing the output impedance as well).  It also makes it very easy to understand volume differences, since the display numbers are actual decibels.  
3) one output line that allows the RCA outs to still work, and one that mutes the RCA outputs.  This allows a number of things: you can play two headphones at the same time, for quick critical comparisons (or listening with a friend), using a sub with your headphone (try it, it's an interesting experience) and auto-muting speakers you may have been feeding from the DAC.
4) ability to be both USB powered, but also have a high power mode that doesn't change the gain (and thus maintains the low output impedance) but adds more headroom.  For many headphones USB bus power is all you need, but it's nice to have that little extra power you can send if needed, and unlike gain switches, it doesn't make things more noisy, or add impedance.  
5) The DAC chip was the same as the DAC chip in my bifrost, which even though I wanted to slim down the form factor, I loved the sound of the bifrost.
 
That all looked great on paper, and combined with Grace's reputation, I decided to jump in.  Even if the amp was merely decent, $499 was a reasonable price for the DAC, given the components and features.  
 
So, I got it; did it live up to my hopes/expectations? (well, spoiler alert if you noticed the rating I gave, you know the answer to this)  
 
Yes, absolutely it fulfilled my expectations, 100% and then some.  
 

 
 
First, the build quality, fit, finish and feel exceeded my high expectations.  Supposedly the engineers at BMW exhaustively test each car, so that the sound and feel of the door closing is just right.  That's how the volume control feels when you turn it or push it down (push to mute or hold to access the settings menu).  The front, back and top plate is one piece of high quality aluminum.  It feels solid without being heavy.  It manages to seem both robust yet elegant.  
 
It's the apple macbook pro or BMW 5 series of audio equipment.  Fit, finish, feel and performance all coming together perfectly.
 
Now, on to the sound part:
 
It manages to be incredibly transparent, spacious, with great treble and bass extension, without sounding dry and brittle.  It's extremely transparent, yet musical.  Nothing about it sounds unnatural.  It just gets out of the way, and takes on the character of the headphones and music instead of imparting its own footprint.  This isn't what you want, if you want to hear your DAC/amp in your chain.  You don't hear the m9XX, you hear *through* the m9XX.  You'll instead hear your music and headphones.  To me, that's the ultimate compliment for a DAC/amp, that it simply gets out of the way and lets everything else do its part.  
 
Now, coming back to what I said at the beginning, where this amp really shines is on lower impedance headphones.  It can drive an HD650 very well, but to me, in some ways, driving low impedance headphones has its own challenge.  Most of the headphones I own are very current hungry, that is, instead of needing a lot of absolute power, they need a lot of current to tighten up and define the bass, and refine the treble and midrange, and give shape and definition to the soundstage and accuracy to the imaging.  This is the real key to the m9XX, that it can make your low impedance headphones sound like you've never heard them before.  And it can drive IEMs well.  And it can even sound really good on a freaking HD800.  0.08 output resistance with 1mW of power makes this ultra versatile.  
 
High Power Mode is vastly superior to most amps High Gain Mode: more current, less noise, no increase in output impedance
 
Talking a bit more about how high power mode works, and why it's substantially superior to the "high gain" mode of most headphones: In high power mode, instead of increasing the gain, Grace simply adds more headroom.  You'll notice that 70 on low and high gain are the same loudness.  You don't get an increase in noise at all, like you do with most DACs high gain mode, you don't get an increase in output impedance.  You do get more current, which most amps high gain mode don't actually send more current, they just increase output impedance.  Basically High Power Mode adds headroom and the ability to use settings over 90 without running out of current on tap and getting a "OC" (Over Current) error and the consequent muting.  
 
Now on to how it sounds with various headphone pairings I tried it with.
 
With the HiFiMan HE400i, the pair is just effortlessly balanced and precise.  
 
I think the HE400i paired with the Grace m9XX could be my endgame setup.  As they've both rolled past 150 hours, with 80 hours of "on head" time, this is just such an incredible pairing.  
 
Ihis is the *perfect* pairing for long sessions.  Enabling crossfeed on the Grace, and this is the least fatiguing headphone I've ever heard.  The resolution and transparency means that I never have to strain to hear anything.  The tonal balance, flat bass and  (to me) neutrality means no frequency ever wears you out. The crossfeed makes for a less fatiguing listen as well.  While the soundstage isn't vast on the HE400i, it's so coherent and imaging is so precise, that it's an effortless listen.  It's present without being sibilant.  The bass is full and extends, without booming (which can get fatiguing after a while to me).  
 
The signature, right away, can almost seem boring.  But it's just so resolving, transparent and fast, while being smooth and musical, that those qualities become fun in their own right.  I realize instead of being like "wow, oh my god the bass" like I am with some equipment, more often I think "wow, this song is incredible."  WIth this pairing it just becomes so squarely about the music as opposed to noticing things about the headphone, amp and DAC.  
 
The word that keeps coming to mind with this pair is simply "effortless" They just effortlessly bring me my music.  They get out of the way.
 
Finally this pairing has by far the most listenable range of volume I've ever heard.  On the Grace, I find the HE400i listenable from 59 to 79.  That's 20 dB of difference that I find them perfectly listenable.
 
With the Fostex THX00, the pair is rich, smooth, detailed and powerful
 
The THX00, like the other TH series cans, and the old Denon DX000 predecessors are tricky to amp, despite not needing much in the way of absolute power.  They're very easy to drive, but if they don't get a lot of current, the bass can be quite uncontrolled, especially on hard fast bass transients.  The Grace comes in and cleans this all up with aplomb.  A can that out of other amps can sound average in terms of detail and imaging becomes quite good in those areas.  To my ears the Grace gives these cans exactly what they need, a very resolving and spacious DAC and tons of current because of the Grace's ultra low output impedance in the amp section.  it's been said by more than a few that it almost sounds like the THX00 was tuned for the m9XX, and it's hard to disagree.  But I also think that almost any low impedance cans with powerful magnets in the drivers (the THX00 is over 1 Tesla) benefit from the current the m9XX pushes.  The m9XX doesn't change the THX00's signature, it just tightens everything, smooths some of the rough edges out and adds a level of detail and transparency.  Essentially it enhances the THX00's strengths (bass power, fun factor) while polishing its perceived weaknesses (detail, bass texture, imaging).  What you're left with is a heck of a fun, yet technically proficient pairing.
 
I also spent about an hour with the TH900 with this pair and the bass on the TH900 with the m9XX was legendary: controlled, powerful, deep, no bleed over.  Everything I said about the THX00 applies here, except with the TH900's even more powerful bass.  If you don't like the TH900's hard V signature, this won't "mellow" it out all.  Again, what the Grace does is get out of the way and lets the headphone do its thing.  If you're a fan of the TH900, you'll love this pairing.  If you aren't a fan of the TH900 sound, you're still going to dislike it.
 
 With the Sennheiser HD650 the pairing is incredibly smooth, detailed and refined with gorgeously textured bass and smooth, if a bit veiled, treble
 
There are two approaches to properly amping the HD650: 1) something that shows the headphone's inherent character or 2) something that provides a little more treble energy to shake off the "sennheiser veil."  If you want the latter, this isn't the pairing for you, because as previously stated what the Grace does more than anything is get out of the way and lets the headphone do its thing.  That means this pairing does much more of the former, you hear how the HD650 really sounds.  It's incredibly detailed, the bass has a sort of refined grace, the treble has a relaxed smoothness that's simultaneously detailed.  To some this sounds veiled and boring, to others it's the pinnacle of engaging treble and a deep soundstage that sounds like you're watching a concert from about the level of the soundboard, as opposed to the front row.  If you want to "shake off the veil" I'd go with something a bit more energetic, like maybe a Lyr (with either solid state or more energetic tubes).  
 
This pairing really shows what the DAC section is all about though, as the microdetail somehow manages to be both precise and crisp yet musical and relaxed.  It's a sort of elegant beauty that sounds really classy.  It's probably not the amp I'd use if I was building an HD650 system from scratch, but I've come to realize that has more to do with some dissatisfaction I personally have with the HD650's relaxed signature.  I wanted an amp that would "fix" it, and as I've already stated, the m9XX isn't going to fix any issues you have with a headphone's signature, it's simply going to reflect the base signature, and refine the more technical aspects.  I think of it like a photographer making a model look as beautiful as possible with skill in photography, without using photoshop to "fix" things.  It brings out the best of what is available.  
 
Grados are frighteningly fast and energetic, gain a bit tighter punch in the midbass, and become an even more fun, if still a strictly technically flawed listen
 
I think there tend to normally be two camps with regards to the Grado sound: those that think it's complete trash that hipsters love and those that can't accept that there might be ANY flaws whatsoever.  I tend to fall into a middle ground.  The SR60e is one of the first three headphones I bought when I got into headphone audio (the Denon D1001 and Audio Technica AD700 being the others) and I'll always be fond of the sound.  They can bring a lot of energy, especially to older classic rock recordings.  They provide a lot of speed on the cheap.  But I also realize that they're honestly technically deficient if the goal is to reproduce what is in a recording.  Again, the Grace won't give them some semblance of sub bass, it's still as rolled off as ever.  The Grace will, however, "perk up" the Grado upper midbass hump, providing classic rock kick drums a bit more thump, thus somewhat equalizing the perception of the signature, without changing the Grado house sound.  Grados are notoriously current hungry, and the m9XX defnitely provides the current they crave.  This results in a lot more refined sound.  I tried the pairing with the SR80e and the SR225e.  The Grace really took the SR225e to a great place.  The DAC section really enhanced the soundstage and imaging.  The amp section gave their already good speed the high octane it needed to become a gloriously fast headphone for metal and prog rock.  The amp section also gave that bass hump some authority and depth that it normally lacks.  The SR80e didn't see quite the same benefit, though you got some semblance of a soundstage (unusual for the SR80e, which to my ears has virtually no depth or soundstage width).  The SR80e's at times flabby midbass tightened up.  The fatiguing upper midrange smoothed out just a touch.  It's not going to turn a SR series Grado into a GS1000, but it will take the Grado house sound to the next level, while staying unmistakably Grado, for good and/or ill.  
 
Other miscellaneous pairings I tried
 
Just going to dump my pairing notes with headphones that are either less common or that I didn't spend a TON of time listening to:
 
1) TheAudio Technica  ESW9A is probably my most difficult headphone to amp, in the sense that most amps tend to actually make it sound worse.  As a headphone it's kind of muddy, has boomy bass, and isn't particularly transparent.  Treble/upper midrange is kind of weird and veiled, but with a little sparkle on the very top.  The pairing essentially brought all these qualities to the fore.  It did add a bit more detail, but overall, yeah, this headphone still sounds better out of an iPhone than it does when amped.  It's pretty though, and very portable.  A good headphone to non-critically walk the dog, not to plug into an amp and listen to carefully
 
2) The Shure SRH840 is probably the easiest headphone I have to amp.  It's so workmanlike that it just does its job with almost anything.  The pairing worked well.  The Grace, as per usual, added a bit more detail retrieval and transparency and firmed up the bass.  If you want a slightly warm, soft U-shaped monitoring style headphone with good treble clarity, this is it.  I love this headphone, but it won't wow you immediately.  This pairing is an ultimate all-arounder when it comes to closed headphones.  It does everything very, very well, but doesn't amaze you with any single aspect.  
 
3) The Audio Technica M50X has issues with bass bloat at times, and the m9XX helped with that.  The m9XX couldn't do much about the lack of soundstage though, it still sounds like you're having sound pumped into the middle of the brain, and imaging is still basically only left middle and right, no nuance.  The m9XX did refine the treble a bit though.  the M50X is a headphone that is notorious for sounding the same pretty much no matter what is amping it, and that held true here as well.  
 
4) The Audio Technica AD700 has some issues with grain, and the m9XX brought that out.  However, the AD700 is also incredibly detailed and spacious and the m9XX highlighted and enhanced that.  At times the soundstage was almost so vast that imaging lost a bit of coherence.  Crossfade helped with that, making for a large (thought not cavernous) soundstage that was very precise imaging wise.  The AD700 is one of the most rolled off headphones in sub bass I've ever heard, and that stayed true with the m9XX.  
 
5) The Koss KSC75 is an ultra portable on ear clip on headphone.  But its quality is incredible for $15.  THere are a lot of $100 headphones it wipes the floor with.  I would never use it on the m9XX on a regular basis, I strictly use it as a portable "walk around outside" headphone.  But here goes anyway: The KSC75 has great treble detail and sparkle, though is fairly rolled off in the bass (mostly due to the way it just sorta hangs around your ear).  The m9XX made it sound even more spacious and detailed.  
 
6) The Denon D1001 is essentially the same headphone as the long running and budget popular Creative Aurvana Live.  I believe Fostex actually makes it.  It has a budget version of the Fostex house sound regardless.  It has pretty great midbass and sparkly treble.  It's certainly a V-Shaped headphone, which helps hide its weaknesses (a bit congested and not super resolving).  I was actually surprised how much the m9XX improved this headphone.  The sometimes boomy bass was tightened substantially, and gained more extension.  The muddiness was cut fairly dramatically.  I could hear some grain, that the muddiness had never even allowed me to hear before.  Grain is always preferable to muddiness, so this was a welcome revelation.  This is another "knock around" headphone.  I've owned it and enjoyed it for about 10 years now and it was nice to have some new life breathed into it.  It's still not an amazing headphone, but the Creative Aurvana Live version is still a bargain (Denon long ago discontinued this, at the same time it notoriously switched from Fostex for the rest of the DX000 serious, leading me to believe this is a Fostex headphone).  But I mean I don't know anybody who would buy an amp like the m9XX for it.  
 
Notes on the filters:
 
The filters are very subtle, so it took quite a while for me to suss out their effects.  They don't so much change the frequency response much, their primary effect is in how they handle the delicate balance of rich, absolute timbral accuracy and maximum clarity.  I don't think people realize that this is often a tradeoff, as super compressed music with lots of treble can often cause aliasing distortion with ultra-high-quality DACs.  I think this is a lot of the phenomenon people notice when they say certain top-shelf DACs are "very harsh to bad sources."  Sometimes it's almost like people here relish a DAC that makes pop music sound like crap, as if that's a badge of pride.  Grace, however, decided to give you the option for a more detailed DAC and a more forgiving DAC (to simplify things greatly).
 
F1: This is given the default spot as the filter, and I think for good reason.  It's the setting that will probably sound best, for most people and how they use the m9XX and the music they'll most use for it.  This is the best setting to use if the Grace is both your DAC and your amp and you're listening to compressed music.  There are two aspects to this setting (like all the settings): fast roll-off and in phase modulation.  The first essentially cuts down on aliasing distortion issues that can happen in highly compressed music with lots of high frequency content.  The second keeps a linear phase response, for maximum preservation of the source signal.  The latter can be bad if you're using an amp that isn't the Grace, ie using the RCA outputs to feed an amp or powered speakers.  Something about how they built the amp section of the Grace seems to do away with the intermodulation issues that can happen with a linear phase.  This intermodulation effect seems to be especially problematic with tubes, which can have microphonic tendencies.  I honestly couldn't tell much difference between F1 and F3, until I put the m9XX in front of my SSHM hybrid tube amp, and there the F3 setting sounded much cleaner than F1.  
 
TLR: if you're listening to typical pop/rock music that is loud with a lot of treble, and you're not using an amp after the Grace, this is probably your best setting.  
 
F2: This is probably the most accurate filter, and what would probably measure the best in objective testing.  It is susceptible to aliasing issues with highly compressed, treble music.  ie pop music will sound somewhat harsher with it.  It can also theoretically have intermodulation issues if it's being used with an external amp.  But when using it with well-recorded, non treble-heavy material, and the Grace as an amp, it provides the ultimate in transparency.  
 
TL;DR: if you're listening to well recorded and mastered music and not using an external amp, this is your no-compromise absolute transparency setting.
 
F3: This setting is the ultimate "fix problems" setting, everything will sound good out of this setting, regardless of what is going through it, and you'll never have to worry about problems.  It sacrifices a bit of complete transparency to cut down as much as possible on aliasing distortion and intermodulation distortion.  
 
TL;DR: use this if you're listening to highly compressed music with very loud treble and using an external amp out of it (especially at tube amp), or if you're experiencing distortion issues and you're not sure what part of the chain they may be coming from.  
 
F4: This is probably the filter that can, theoretically, get the absolute best sound out of the m9XX's DAC section.  It produces the full, rich tone, without giving worry to aliasing issues in high treble energy music, but shifts the ringing to reduce any intermodulation effects that might happen with an external amp (especially a tube amp)
 
TL;DR: use this if you primarily listen to exceptionally well recorded music and are using an amp (especially tubes) after the m9XX.
 
Conclusions: 
 
This is an INCREDIBLE piece of gear.  It's so versatile that it's astounding.  I'm not an IEM guy (can't stand the feeling of having something in my ear canal) but it can effortlessly amp those with complete silence, and it can drive an HD650 very well.  Heck, several people even say it's a great pairing with the Sennheiser HD800.  It's so well designed from all angles: look, feel, build quality, feature set, sound quality and just some indescribable "ahhhhhh" feeling of satisfaction every time I look at it.  Where it really shines are the current hungry high end but low impedance headphones that have really been a growing market segment these days.  The crop of easier to drive planars and low impedance dynamics will sound better than many thought possible through the m9XX.  It's not going to blow you away on a first listen though.  It's not going to grow a subwoofer in your bass deficient headphone, pr give a non-detailed headphone HD800 level resolution.  But it will take the headphone that it's being given and shine it as well as it possibly can.  It's not going to power a HE-6, but it can handle almost every "normal" headphone with aplomb.  I'm pretty sure this is the last DAC or amp I will ever own, unless there's some crazy leap forward in technology.  
 
I think of it this way: If this thing was just a DAC, and had no features (no crossfeed, no filters, no volume control, etc) it would be a bargain at $399.  If you had a unit that added the features it has (crossfeed, filters) for just that add on unit, you could charge $100.  If it was just a solid state amp, given its ultra low impedance, the way the volume control and high power mode work, the cleanliness, just the amp section would be a bargain at $250.  Put that all together and you have a product, that on pure sound quality alone would be a bargain at $750.  Then when you combine the convenience, elegance, and quality of the build and form factor, this is easily a $800-$1000 product.  It would make a lot of "best value" lists if it cost $899.  Grace and Massdrop got it to us for $499, which makes it a stupefying value.  
 
You can throw this in a messenger bag, without any worry, and you can have sound quality that will put many $2000 systems to shame.  You can power everything from an IEM to a HE500 and never feel that your gear is being compromised.  It's an all around solution that somehow manages to make no real compromises at a budget friendly price.  If your headphones don't require s small scale nuclear power reactor to drive, I have one piece of advice about this:
 
If you have the chance BUY THIS THING

Ever since its debut, I’ve been a big fan of the AKM 4490 chip in almost every device I’ve heard it in.  The Bifrost 4490 was a great midlevel DAC, I love the m9XX and the Modi 2 Uber with 4490 is an incredible value.  So, when I saw the Cayin iDAC6, I was extremely intrigued.  First, it was the only implementation I had seen that used dual AKM 4490 chips.  Secondly, it had the option of going fully balanced, and also having the ability to choose between tube and solid state analog output stages.  
 
I bought my unit from another Head-Fier used, who got his unit in one of the very first batches that were sold to the public.  He kept the amp, but after A/Bing his unit preferred his Yggdrasil to the iDAC6.  Given that Yggy is a $2000+ DAC that is generally considered the best DAC in the world, that’s not much of a slight on the iDAC6.
 
Gear used:  Unless otherwise noted, most testing was done with my TorpedoIII amp and HD800 headphones, unless otherwise noted Sonarworks calibration was applied to the HD800.  Inputs used were a mix of optical, USB and USB via Schitt Wyrd USB decrappifier.  
 
Looks:
 
The iDAC6 is a pretty classic looking medium/large DAC.  It’s appointed in a matte, brushed aluminum type finish, with a chrome polished knob and three chrome function buttons.  The volume on knob had a very tasteful white LED ring around it to indicate that the device was powered on.  A very standard LCD screen indicates necessary info like sample rate, mode, level, input type, output type, etc.  The LCD is slightly angled upwards, which makes reading it a bit easier since you will normally be looking slightly down at it while adjusting it.  A very subtle white screen printed cursive Cayin logo sits unobtrusively in the top left corner.
 
All in all, I would call it a very classy looking DAC that doesn’t really make any statements one way or the other looks wise.  It looks very typical of silver finished equipment.  
 
While not being larger than most DACs of its class, it is quite heavy.  I have mixed feelings about this, in that it’s no worse in this regard than competitors, but I also just don’t feel like DACs need to be this big any more.  This is doubly problematic because it runs so hot.  TO me the traditional argument for DACs being this large and heavy was because they needed to be that big to properly dissipate heat.  But this DAC still runs very warm.  Not unbearable to the touch or anything, but warmer than any tube amp I’ve ever owned.  In comparison to some of the newer top quality DACs I’ve tried, the footprint is quite large.  And as noticed during the tour, shipping for this item can be insane because it weighs about 15 pounds.  That being said, the size and heft does give it a feeling of being substantial.  Overall I’d call the size and heft a mild negative.
 
Features:
 
The iDAC6 features a full range of the normal selection of inputs and outputs to be expected of top range DACs: Inputs are: Optical TOSLINK, Coaxial TOSLINK, AES, USB Type B. Outputs are: Balanced XLR dual 3 pin, unbalanced RCA.  Output is on the higher side (2.2V RMS unbalanced, 4.4 V RMS balanced).  Input is selected by pressing the small chrome “source” button.  Output does not need to be selected, as they both are always active.  If you have amps plugged into both the balanced and unbalanced outputs, they will both receive signal from the DAC.  As far as I know there is no way to switch between them, other than simply unplugging from the output you aren’t using (or turning off the amp receiving that signal, obviously).
 
On USB DSD 64 and DSD 128 are supported natively, while up to 32/384 are supported through PCM.
 
Five filters that are native to the AKM 4490 chip are use selectable.  This was very interesting to me, because although I’ve had several DACs that use this chip, this is the first time I’ve had one that just used AKM’s filters unchanged.  The Grace m9XX only uses 4 filters, and as best as I can tell they’re slightly altered versions of the 4 AKM filters.  The Schitt products that use the AKM4490 do not allow the user to select the filter, and seem to use the Slow filter always.  The f filters are: Super Slow Roll Off, Short Delay Slow Roll Off, Slow Roll Off, Short Delay Sharp Roll Off, and Sharp Roll Off.  Here is a picture that AKM has released describing them:
 

 
This picture somewhat exaggerates the differences between the filters, which can be quite subtle.  Unlike the filters on the Grace m9XX, they are a bit more noticeable here.  I found slow roll off to be the best sounding, and unless otherwise noted, the rest of this review will be written about the sound with this filter employed.  Much of choosing the right filter will be based upon the music you listen to and your downstream amplifier, as one of the primary functions of these filters are to deal with issues in aliasing distortion and intermodulation distortion that can arise with some music in some setups.  This isn’t really the place to go into a ton of detail about how the filters work, but I will just say that most sources seem to agree that slow roll off is the best setting if you aren’t having issues elsewhere in your chain with IM or aliasing distortion.  The different filters can be switched by pressing in the large rotary knob.
 
In addition to the five filters, the iDAC6 also allows you to choose between solid state and tube output stages ****IF YOU ARE USING THE UNBALANCED RCA OUTPUTS****.  Make sure you are clear on that.  If you are using the balanced outputs, you are limited to the tube output stage.  When plugged into the RCA unbalanced output, you can switch between tube and transistor output by hitting the “timbre” small chrome button.  If you do this when plugged into the balanced output the iDAC6 will always output from the tube stage.  Even if the LCD says “transistor” if you are plugged into the balanced output you are getting the tube output stage.  Because of this, the tube stage is always on whenever the iDAC6 is on.  Even if you are running it in transistor mode through the RCA output, the tube output stage is still running.
 
The iDAC6 also allows you to alternate between two operation modes: pre-amp and line out mode.  The differences are thus: line out mode allows for the playing of DSD natively and pre-amp mode allows you to adjust the unit’s output level digitally via the AKM4490’s built in 32 bit digital volume attenuator.  If you want native DSD, you must use the line out mode, as pre-amp forces everything to go to PCM, since it needs to be PCM in order to do the digital processing that the volume control uses. 
 
So, now that the thorough description of the look and features, on to the meat of the review. 
 
Sound: 
 
The first thing I notice about the sound of this DAC, is the “heft” of its output, especially in the bottom end.  Comparing it to other DACs I own, with a given amp, it has very clearly the most punch and impact of anything I’ve ever tried.  I believe this comes from the output stage, rather than the chip/conversion stage.  The Grace m9XX, when run as a DAC only into my TorpedoIII is nowhere near as punchy as the iDAC6 through the RCA output is into the same amp with the same headphones.  As both of these utilize the same AKM4490 chip (albeit with the iDAC6 sporting two of them), I don’t think this can be a tonal characteristic of the chip.  It is even punchier than the Bifrost 4490, which is known for being a punchy DAC.  It just has an effortless sense of power when powering through transients.  At times, in certain setups, with certain headphones, it can sound almost “hard.”  This is especially evident when comparing it to the Modi Multibit, which has a sort of softness of sound to its output.  If you are trying to get a bit more “oomph” in your chain, this may very well be the DAC for you.  But if you have a super punchy amp and headphone, it may border on being too much.  There were times with the TorpedoIII and HD800 (both very punchy the way I have them setup) that the setup was so dynamic, that it was like it almost overwhelmed my ear and I was unable to hear detail in transients.  It’s not that the detail wasn’t there, I don’t think, it was that the setup was just too much for my ear.  It could be a bit fatiguing in this setup.  This held for both the tube and transistor output, although I think the transistor output was a bit less so, especially on bass transients.  The iDAC6 in tube mode could really produce some absolutely thunderous kick drums, for example, when paired with a punchy amp and a very lively headphone.  
 
With an HD650 though, this characteristic really shook off the Sennheiser veil, almost more than I’ve ever heard an amp doing.  Normally we think of needing just the right amp for the HD650 to come alive, but pairing it with the iDAC6 allowed some amps that I normally wouldn’t think of as being a good match with the HD650 to really shine.  The HiFiMan Edition X also really benefitted from this, as it’s characteristic “soft” sound was woken up just a bit, and while it retained its tonal character, it was a bit more lively and less “soft” sounding.  With the THX00, bass was even more addictively fun.  Drums sounded like thunder.
 
The second thing I noticed about the sound of the iDAC6 is a sense of easy spaciousness.  I wouldn’t say it sounds airy in the way that some Delta Sigma DACs can sound like you’re in a large, cold auditorium.  It often sounded like you were in a large room, but during summer.  I never felt claustrophobic, but I didn’t get a sense of airy spaciousness either.  Again, this characteristic held for both output types, but was most evident with tube output.  Soundstage was both a touch wider and deeper than other AKM4490 based DACs I’ve tried (m9XX, Bifrost4490, Modi 2 Uber 4490).  It doesn’t compete with the Yggdrasil, GuMBy or even MiMBy in terms of spaciousness, but acquits itself very well in that regard int he world of top end Delta Sigma DACs.  I never found myself wanting for soundstage with this DAC, but I have heard larger.  Imaging accuracy was excellent.  Separation was excellent.  It reacted well to when I turned on GoodHertz CanOpener for my crossfeed needs.  I have heard DACs that screw up the HRTF functions that CanOpener applies, somehow.  
 
Micro-detail was very good, but not truly class leading.  To me, this is where even the best Delta-Sigma falls short of R2R DACs.  They, at some point, seem to have to make a tradeoff between harsh and brittle tone or smoothing over some micro-detail.  While the Yggy, GuMBy and even MiMBy could preserve detail without having to be harsh, it seems the iDAC6 had to make this choice and opted for smoothness.  If this choice has to be made, it is certainly my preference that it go in the direction Cayin/AKM chose, which was smoother.  I abhor many of the Sabre based DAC that attack your sensibilities with brittle faux micro-detail.  It’s like many Delta Sigma DACs, because they’re missing some of that micro-detail pretend its there by making the tone harsh.  It’s sort of like when a photographer missed focus by an inch or two, and artificially over-sharpened in photoshop to make up for it.  The AKM4490 based DACs instead just smooth it out and make it sound great.  This has been a characteristic of every AKM4490 based DAC I’ve ever heard.  If anything the iDAC6 has the best micro-detail of any AKM4490 based DAC.  But there are DACs that are a bit better in this regard.  An example of how this plays out, is that you can hear tiny differences in the character of the reverb on some tracks with the R2R DACs, that were just a bit smoothed over with the iDAC6.  Now, this difference in detail is subtle.  I couldn’t even pick it out on an HD650, HE400i or Grado SR225e.  But on my HD800 it was there.
 
On the question of tube output vs. transistor:
 
To me, the feature that will garner the most headlines and confusion about this DAC is the tube output stage vs transistor.  There aren’t a whole lot of tube output stage DACs out there, and I don’t know of any that also let you choose transistor output stage.  To me, the tube output stage almost always sounded slightly better.  It lacked a certain edginess in transients that transistor had.  While the transistor output measures as having less THD, one of the things I’ve learned over the years in audio is that total amount of THD is less important than how that distortion breaks down.  The transistor output seems to have most of its THD located randomly in “harsh” high order harmonics.  The tube output stage (which operates *after* the transistor stage and acts more as a tube buffer) adds just a touch of low order harmonic distortion, which has the effect of smoothing the harsh edginess, just a bit.  I slightly prefer the tube output most of the time.  Think of it like this DAC basically has something like an iFi micro iTube built in.  As this costs $329 on its own, it’s quite a coup to have it built in and switchable.  In systems with no tube stage anywhere in the signal path I always preferred tube output stage.  But even in systems that had tubes otherwise in the signal path (TorpedoIII, for example) I still generally preferred the tube stage output, though I was able to appreciate the transistor output on some recordings a bit more.
 
 
Comparing to other DACs I either own or significantly demoed:
 
Grace m9XX: iDAC6 does everything the m9XX does except crossfeed just a little bit better.  For anybody who is familiar with the way the m9XX sounds, think of the iDAC6 as like an m9XX on steroids.  A bit more spacious (I believe due to operating two AKM4490s in dual mono mode), a bit more dynamic, a bit smoother, and a bit more detailed.  They’re entirely different products, with different usages obviously, but I’ve had a lot of m9XX owner’s ask me to make the comparison.  A lot of m9XX owners who bought the m9XX as their first high quality amp/DAC might eye the iDAC6 as a potential upgrade path for a full sized desk top rig, allowing the m9XX to serve as a smaller, semi-portable secondary option.  
 
Schiit GuMBy: this is the most clear tossup.  The GuMBy is a bit more detailed.  They both have similar tonalities though, leaning just a touch warm and inviting.  However, iDAC6 is more dynamic.  Bass transients (think kick drums, bass slaps, etc) have an additional weightiness to them that GuMBy lacks.  The iDAC6 hits harder, the GuMBy is a bit more detailed and spacious.  Which you prefer will depend on personal preferences and also your system.  I could absolutely see somebody with, for example, a HiFiMan HE1000 or Sennheiser HD650 based system preferring the iDAC6, to really make it hit harder and faster.  I could see somebody with an Abyss or HD800 preferring the GuMBy, not needing the additional impact, but being able to reproduce the extra detail.  It will be down to not just headphone, but also amp.  At this level, it really does come down to system synergy as much as anything.
 
Schiit Yggy: Yggy is better.  It has the detail advantage that GuMBy had, but also has the additional power and impact that iDAC6 has.  Now, the Yggy isn’t as tonally versatile as the iDAC6 is, so if you’re a tinkerer, you may still prefer the iDAC6.  But if you’re simply after the best single sound possible from your DAC, the Yggy wins out.  It does everything the iDAC6 does, but adds in a bit more space and detail.  That isn’t a slam on iDAC6 is, Yggy is over twice its price.
 
Overall final thoughts
 
I like this DAC a lot.  I bought it.  It’s my favorite Delta Sigma based DAC I’ve ever heard, by a pretty fair margin.  I’d call it highly dynamic, very detailed, spacious and neutral with maybe the slightest hint of warmth.  While it is dynamic, the smoothness keeps it from being fatiguing, unless it’s also paired with an exceptionally punchy amp and headphone.  And even then some people may still love that sound.  With most amps, it adds an often looked for sense of punch, smoothness and warmth compared to most other DACs in its price range.
 
In the end, I think the $999 price is fair.  It’s not in the land of stupendous values, like say, the Schitt Modi Multibit.  But it certainly isn’t overpriced at all, you get what you pay for and a little bit more.  I think this would be fairly priced up to about $1500-1700.  So, it’s a good deal at $999.  There isn’t another DAC at $1000 or less that I think clearly beats it.  Some DACs offer something different, but nothing that I can look at and say “this is clearly better.”  It’s not until Yggy that I can point to something and say “yeah, that’s just definitely better.”
 
For me, personally, I am considering selling mine.  Not because I’m disappointed with it at all, it just sits in a weird position for *my* setup.  With my amp, it’s a touch more dynamic than I really ideally want.  The TorpedoIII amp in my system recently had an upgrade called output constant current supplies; after this upgrade, the TIII took a fairly massive leap forward in quality and became a more dynamic, harder hitting amp.  Before this upgrade the iDAC6 was ideal for my HD800 rig.  After, I wanted a tiny bit more detail, and a tiny bit less dynamicism out of my DAC.  This has me leaning towards GuMBy.  I currently have the iDAC6 in my secondary setup feeding the HiFiMan EF2C, which is a bit of an odd pairing.  My main pairing is the Schitt Modi Multibit feeding the TorpedoIII.  My long term move is likely moving the ModiMultibit to the secondary setup and side-grade moving the iDAC6 and buying a GuMBy.  I think the GuMBy’s softer, slightly more detailed profile may work better in *my* setup.  However, again, I want to stress this isn’t a want for quality, but a bit better system synergy between DAC and amp, given my headphones.  To be fair to iDAC6, I even thought Yggy was a bit too dynamic in my setup, GuMBy seemed *just right*. (thanks to Sorrdje for helping me think through this, btw).
 
That all being said, I also like this DAC so much that part of me is considering keeping it, just in case my future system takes another direction, and I again want this slightly warm, very detailed and spacious, highly dynamic sound from a DAC.

 
 
Intro - Context for the purchase (Read this if you want to understand where I am coming from when relating to this headphone.  I like to create a narrative to my reviews, but strictly speaking this portion isn't actually about the headphone, but rather helping to understand my frame of reference).  
Unboxing/Accessories:
Aesthetics/Pads/Cable:
Amp Pairings:
Burn-in/Isolation:
 
So… Finally… The Sound:
 
Starting with the various parts of the frequency response:
 
Bass (9.5/10 overall) - quantity 9.5/10, impact 9.5/10, texture 9/10, control 8/10
 
 
 
Midrange 6/10 - (smoothness 8/10, clarity 7/10, linearity 4/10, lower 8/10, upper 4/10)
 
 
Treble 8/10 - (presence 8/10, clarity 7/10, extension 7/10, airiness 7/10, smoothness 9/10)
 
Other aspects of the sound as a whole:
 
 
Conclusion:
 
This is a truly great, refined, basshead headphone.  It doesn’t do anything poorly.  It does several things extremely well (bass and smoothness).  It does have some slight weaknesses (wonky dip in the in upper midrange/lower treble, not the strongest imaging).  It’s good, but not great in most other areas.  While it’s not the final word in resolution, it won’t offend there either.  It’s good looking, I find it comfortable.  And more than anything it’s a fun headphone as long as you aren’t expecting reference level neutrality.  It is an absolute steal at $399.  It’s a $700 headphone, at least.  It would be a bargain at $700, in fact.  It has class leading bass performance (I don’t consider the TH900 or any Audeze offerings as being in its class, costing nearly 3X as much).  My only hope for this headphone was that it would get back some of the joy I experienced with the D7000, and I'd say it succeeded there, and then some, for roughly half the Price of what the D7000 originally cost.  
 
A Note about EQ:  Eq can largely repair the mid-range issue, if you so desire.  To give you a rough idea of how I EQ these, here is a simplistic adjustment I sometimes use when I use iTunes.  It retains the overall character of the headphones (none of the cuts are more than 3dB), but irons out a bit of the wonkiness, and makes the bass a bit more linear, while still remaining squarely a basshead can.
 
 
 
Head-to-head comparisons I was able to make with somewhat comparable range headphones:
 

I was looking for a work solution to drive my relatively low impedance headphones I use at work (Shure SRH840 and Grado SR225e), while also being able to feed my powered desktop speakers (BOSE companion II).  As I don't do critical listening at work, I didn't need the highest level of audio quality, but I needed something small and portable so that I could easily put it in my briefcase when leaving for the weekends, etc.  I also wanted it to look okay, but have a fairly small footprint.  I looked at a lot of products and ended up trying this and the HiFiman EF2A.  They both seemed to fit my needs of a AMP/DAC combo in an attractive, small package.  
 
My expectations were that the HiFiMan would sound better and look better on my desk while the Audioengine would be more easily transportable.  
 
While the HiFiMan did look somewhat better, it looked kind of ugly from behind, and didn't have a convenient spot on my desk.  The Audioengine on the other hand, while not being a talking piece, was much more convenient on my desk, and its design aesthetic gelled well with my Macbook Pro, Bose desktop speakers and the headphones I was using it with.  The HiFiMan looked somewhat out of place.  
 
Soundwise, I expected the HiFiman to sound better, but that turned out to not be the case at all.  If anything, the humble D1 blew the HiFiMan away.  First, with low impedence, easy to drive headphones, the HiFiMan was mostly useless, as it had major channel imbalance issues up until you got past 1/4 power.  However, for easy to drive headphones, much past 1/4 power was at earsplitting levels.  The d1, on the other hand was perfectly balanced at all levels.  Sound on the D1 cut out when it was below 8 oclock (I know there aren't markings on the D1, I'll explain below), but was completely balanced and noise free at all levels.  The HifiMan, on the other hand, had an extremely narrow band where it was usable, and you had to constantly adjust it based on the recording.  
 
Further, given the headphones I was using both had 1/8" terminations, I decided that there was no use even spending any more time with the HiFiMan, and sent it back after just two days (I had originally planned to spend a week with each and maybe even keep both).  
 
As the D1 burned in, the sound continued to gain more spaciousness and the bass tightened up.  At this point I've logged about 100 hours on it, and the sound seems to have stabilized.  
 
Now, the first con is that the volume knob had no markings on it whatsoever, making it hard to see what your volume level was at, at a glance.  Especially given that I will sometimes move this around, it was easy for the volume knob to be turned way up, without knowing.  However, I solved this by simply pulling out a sharpie and making a small dot on the knob when it was turned all the way down.  I put the mark at 7 o'clock when it was turned all the way down.  After doing that, I found that with my SRH840 and SR225e, 9 o'clock was my normal listening level.  8 O'clock could be used for soft listening, 10 o'clock for "slightly loud fun."  After about 12, things were uncomfortably loud on those headphones.  
 
With the HD650, 12 O'Clock was normal listening level.  However, with the HD650, is where the limitations of this guy started to show up.  While the D1 would get the HD650 loud enough, it didn't have the same bass power as a full desktop amp with a dedicated power supply.  The HD650 was a tick slower than when it was well powered.  It wasn't bad sounding by any means, but the USB powered supply just couldn't give the HD650 quite the juice/headroom it needs to really shine.  However, I never bought this to use with the HD650, which is my go to at home critical listening phone.  Just thought I would point that out.  With tough to drive headphones, it's merely "good" and not superb.  Would I hesitate to use this with the HD650? Not at all, it still worked fine with them, and did dramatically improve them vs. no amp.  But, there was certainly a noticeable difference with a nice, fully powered amp.
 
A further benefit of the D1 is that when you plug your headphones in, it cuts the non-amped out.  So, you can plug in your desktop powered speakers and make use of the DAC and volume knob on the D1, and not worry about turning your desktop speakers off when plugging in your headphones.  
 
Now, as to the sound, with the headphones I use this for, the SR225e and SRH840, it really tightens up the bass.  That's the most noticeable benefit, you get fast, punch bass that's not quite there when those are straight out of the macbook's headphone jack.  Further, there is a smoothness to the highs that really is pleasant.  The midrange is rich.  Soundstage is slightly expanded, you get a nice depth to the vocals.  You can hear when singers even slightly move toward and away from the mic.  
 
It is also dead completely black quiet, no noise whatsoever.  Should be a given, but with sub $200 DAC/amp combos, that's unfortunately not always a given.  
 
In a lot of ways it kind of has an almost tube like warmth, without the noise, difficult load management and unreliability you often get with tubes.  
 
It's small size also allows me to easily take it home, or lock it in a drawer at work, thus have complete peace of mind with leaving it at work.  
 
I can't believe how good this thing is, given the size and price.  I'd happily pay $300 for it, but am sure glad I didn't have to.  It was exactly what I was looking for features wise and sounds incredible.  I in fact bought a second one for at home, and am considering adding a few other audioengine products (their bluetooth transmitter) because of the positivity of this experience.  
 
Is it a perfect amp?  Of course not, don't be silly.  But if you are looking for a quality amp for headphones that aren't extremely hard to drive, it's great.  It provides plenty of current that lower ohm headphones need.  It has a great DAC and a fun sound.  In a sleek, quality build.  For sub $200.