Grace Design m9XX compact USB DAC with integrated headphone amp

General Information

Exclusive design commissioned and co-designed by Massdrop, taking the far more expensive Grace m920 and repackaging it in a smaller/more affordable device

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100+ Head-Fier
Pros: Small footprint, crossfeed is well done, detailed yet fun
Cons: Strange hum when muted, still white noise on sensitivce iem's (minimal)
Grace Design m9XX Review
Where I say stuff that you may want to know about myself and my music preferences
There is already many very well done reviews of this product already here on Head-Fi, so I will try and not belabor my points to much for this review, but you may need to put up with a little of my grandiloquence at times! (Got to keep it interesting right?)
First off I would like to describe my music preferences.  I think this is an important thing to consider while reading reviews as sound preferences are very unique and personal.  I tend to enjoy a more neutral to warm sound, however I do not like extremely rolled off treble.  I need a little sparkle in my headphones or they just sound boring and bland.  My current favorites are the SRH 1540’s.  They have slightly elevated bass and a wonderful treble extension. 
I listen to just about every music genre.  I mostly listen to classical, singer song-writer, scores and electronic/dance.  Yes I do realize those are both large, and widely varying categories.  Point being I listen to many types and styles of music. 
Full sized headphones used to assess the m9XX were the Shure SRH 1540, Beyerdynamic DT 990 pro (250ohm), and the Beyerdynamic T70p.  IEM’s used were the Shure se215, Thinksound Rain2, and RHA s500i.
Overview for m9XX:
Get to know the m9XX on a more personal level
This is one solid piece of hardware.  Prior to the m9XX I used the Fiio e17+ e09k combo for my desktop.  One excellent thing about the m9XX is that it has such a small form factor.  It really takes up less space than my Fiio, which is impressive as that is pretty small already as far as desktop units are concerned.  It is very sturdily built.  It has a good heft in the hand and certainly gives the feel of being premium. 
The m9XX contains two USB inputs.  One has a dual purpose of providing both power (low only) and data transfer.  The second is strictly for power.  Plugging in the second cable enables High Power Mode which will allow you to drive most headphones.  In addition to supporting USB audio (both class 1 and 2). You can opt to use the Toslink input as well.  Finally it features an RCA out allowing you to either connect another amp of your choosing, or to connect to active speakers (will not drive passive speakers). 
Moving to the front of the unit you will find two ¼ jacks for a headphone of your choosing.  It is nice to have the option of two in case you wish to share with a friend, or perform quick A/B comparisons of different headphones.  Additionally, you will find a small 2 digit display.  This little display will tell you all you need to know about what, and how the m9XX is functioning.  I was very impressed with how simple the interface was, so kudos to Grace Design and Massdrop for designing an excellent interface.
The owner’s manual is a wealth of information so I will leave a link for your reading pleasure!
Here is another link to other, more in-depth information for the m9XX.  You will need to copy and past it, as it wont allow me to create a link for some odd reason.
Listen to that music!
If I had to describe the sound characteristics of this device I would say it is near neutral with excellent clarity and refinement.  Distortion is noticeably lower than my old Fiio set-up.  It definitely smooth’s the upper treble a bit, which dramatically decreases some grain found in some tracks.  Furthermore it has a nice tight bass response.  It is very noticeable on the SRH 1540 which can occasionally suffer from bass bloat in the mids.  This has been reduce and has strengthened the overall clarity.  Overall it has a wonderful, easy to like sound signature.  Having said this it may not pair well with certain headphones according to some reviewers, but I cannot comment on this as it seems pair well with all of my cans. 
There are two other important features about this device’s sound.  The first being the filters.  Grace Design describes the filters like this:
“DAC FILTER Changes the response of the digital filter. Push and release the encoder to scroll through the filter response modes:
 F1 = sharp roll off, linear phase
For linear phase response and time coherency. Fast roll off protects against aliasing distortion from high amplitude high frequency content.  Best for recordings that are loud, compressed, and with lots of treble. Will contain substantial ringing before and after transients (pre-echo and post-echo). Note that the ringing occurs at the Nyquist frequency (½ of the sample rate), so it is not directly audible. However, it can cause intermodulation distortion in downstream components.
 F2 = slow roll off, linear phase
For linear phase response and time coherency. Best for acoustic music without compression and artificially high levels of treble. Will have very low levels of ringing before and after transients but is susceptible to distortion artifacts caused by high amplitude high frequency information in the program material.
 F3 = sharp roll off, minimum phase
Not linear phase in the pass band. Fast roll off protects against aliasing distortion from high amplitude high frequency content. Best for recordings that are loud, compressed, and with lots of treble. Will contain substantial ringing caused by transients, but all of the ringing is shifted to after the transient. This can reduce the perceived effects off downstream intermodulation distortion due to the Hass Effect.
 F4 = slow roll off, minimum phase
Not linear phase in the pass band.  Best for acoustic music without compression and artificially high levels of treble.  Will have very low level of ringing caused by transients and ringing will be shifted to after the transient.”
Truthfully they all sound very similar and I was not able to tell much difference between all of the filters.  I decided to leave it at F1.  It paired best with my SRH 1540’s simply because it smoothed the notorious treble spike it has.  I do think the name is misleading however.  The name “Sharp roll off” seems to imply that the treble is going to drop off the face of the earth and be non-existent.  This is not true.  Instead it is simply smoothed and given less edge.  This was also helpful for my T70ps as they are exceedingly bright at times.
The second aspect that I really find myself enjoying is the crossfeed feature.  Some of you may be wondering “what the heck is crossfeed?” Crossfeed is really an ingenious way to help eliminate the polarization that occurs while listening to music through headphones.  When you listen to music through speakers both of your ears hear the music, but one ear hears the signal a little louder than the other (the one that is closest to the source).  Using these varying signal inputs your brain can calculate the special awareness of the music in the room.  By applying crossfeed you enable a similar effect, but with your headphones.  It is employed very well and truly does make for a much more natural listening experience.  Most noticeable changes are: Increased sound stage, better separation, and better imaging.  The only drawback is it has the potential to suck the mids out a bit.  This is only a problem with extremely U shaped headphones, like the DT 990.  With crossfeed on the mids were sucked way out, making vocals sound faint and muffled.  In contrast both the Shure 1540’s and T70p’s (both more midcentric) sound sublime with crossfeed enabled.
Better than the energy drink!
I won’t talk too long about the amp section, as they are not an area I feel well versed in, and there is much discussion already here on Head-Fi in the forum.  What I can say is that it powers all of my headphones exceedingly well and has a velvety black noise floor thanks to the ultra-low output of 0.08 ohms. This includes sensitive IEM’s as well, although there is white noise present, it is very negligible and not noticeable at all while music is playing.
Meant for a desk? Or for the road
This device is certainly compact.  Because of this compactness many have decided to use it as an on-the-go companion.  For me personally it is not a viable simply because I have other portable options that are far more compact (Fiio K1).  While they do not offer nearly the level of audio quality that the m9XX offers, I am willing to sacrifice for more portability. 
Having said this, the m9XX can certainly be used as a portable.  This is because of the ability for it to be driven without a power adapter.  This does limit you slightly as hard to drive headphones will not work as well, however these types of headphones are rarely used as portables.  So, if you are looking for an al-in-one solution for both a desktop, portable, and was the best from your music the m9XX should certainly be on your short list.  It is essentially a far more portable version of the Schiit Bifrost. I do want to clarify though, I have never personally heard the Bifrost.  I make this statement for a few reasons.  One it has the same chipset so this would mean they may have similar characteristics (although this is not always true).  Secondly, I make this statement based on several other users stating they have similar, albeit different characteristics when it comes to quality.
Weaknesses of the m9XX
Everything has a weakness…
Over my course of having the m9XX I have really only found one weakness that I truly do dislike.  The easy mute feature is nice as it allows the user to quickly silence the music.  What I noticed, while testing with my t70p and with my IEM’s was that there is an audible hum noise (almost like interference) when initially pressed.  What is more interesting is that this humming disappears once the screen is dimmed, assuming that feature is enabled.  If it is not enabled than the hum persists until the music resumes being played.  What is concerning is that it seems directly tied to the display, which means it is potentially causing interference any time it is on.  I have yet to read anyone else having this issue so it is possible that it is strictly my device.
Luckily, however, it does not seem to affect the music whilst it is playing, as I tested this with quiet tracks.  I cranked the volume and there was no hum until muted.  My thought is that the hum is somehow tied into the mechanism that causes the screen to pulsate, indicating the music is muted. 
I contacted Grace Design about this issue and the told me this:
  When the device is muted, the display uses a PWM scheme to make the screen pulse. The noise you are hearing is a by-product of this. This noise is usually only heard on very sensitive headphones like IEMs. When the unit is un-muted, there should be no noise caused by the display. This is normal behavior and should not be cause for concern. Please let me know if you have any questions!

Final Verdict
So what do I think?
I personally think this is an amazing device.  Given its price point it is both an investment, but also a bargain.  The only real improvement a person could make would be in the amp.  This is truly only necessary if you have hard to drive headphones (above 300 ohms).  It has wonderful music separation and greatly reduces distortion in the upper treble region.  It will also tighten up the bass response allowing for a nice, tight, impact.  It has been known not to pair well with some headphones, but for me it pairs well with all of my headphones, so long as I take crossfeed off when listening to the DT 990’s.  All I can ultimately say is: Well done Grace Design and Massdrop for bringing a wonderful device to the audiophile community!
Questions? Comments? Please let me know!


New Head-Fier
Pros: Compact, somewhat portable, incredible sound, and for $499.99, it might even best some of the $1500+ DAC's out on the market.
Cons: Only 500 units from Massdrop, and only one drop so far. Relatively new, so limited reviews.
This is my first user review, and I just got the unit a few hours ago, so I'm keeping it short for now, and I'll update more later.




The Grace Design m920 came in a few hours ago. I'll probably end up writing more as the days go by.


Switching between my Grace Design M920 (24-bit 196kHz), M9XX, (32-bit 192kHz) and SMSL M8 (32-bit 192kHz, paired with an Objective 2 amp). The SMSL M8 for the $149.99 drop, is decent, but still doesn't compare to the Grace Design M920 and M9XX. I prefer my digital filters with a Slow roll off (linear phase). With this digital filter setting, the SMSL M8 won't go slow enough for my tastes. Listen to any acoustic vocals song, and you'll notice the difference. The M920 and M9XX handles it very well.


Currently, I'm comparing with my AKG K7XX at level 72 (and for the Objective 2, getting it close to that level as far as I can tell). I'll be switching to different headphones to test even more.


I am really trying hard to find the differences between the M920 and M9XX, but they are so subtle, it's difficult to pinpoint. Listening to Yo-Yo Ma's Cello Suite No. 1 in G Major (via Tidal HIFI), starting at the 1:52 minute mark, the crescendo and drop off actually sounds better with the M9XX than the M920. Yeah, weird, right?

I'm starting to feel a little bummed that I dropped $1500 on the M920, lol. I'll need to keep listening to different types of music so I can feel better about having the M920 over the M9XX.
Asking around the forums for anyone that had both the M920 and M9XX, I came across the same sentiment, and I was able to pinpoint in more detail the difference between the M920 and M9XX.
Listening to "Serve the Ego" by Jewel (via Tidal HIFI), starting at 0:09 and ending at 0:14, you can definitely hear a difference. The M920's lows resonates a bit deeper. I mean, I really, really had to listen to it over, and over. Switching between the F2 and F4 digital filters on the M9XX, I wasn't able to discern any noticeable differences between the two settings.
Which makes sense as to why Yo-Yo Ma's Cello Suite No. 1 in G Major the day before sounded better with the M9XX. A faster paced routine like that is going to sound better with the M9XX, but something slower paced, the M920 will resonate much deeper into your soul.
To be continued...
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looking forward to your updates, please post asap!


Headphoneus Supremus
Pros: DAC transparency; ultra-low-impedance amp; neutral, flat, clean sound; crossfeed; DAC filters; build quality; form factor is small and well designed
Cons: won't drive the most demanding headphones properly (HE-6, 600 Ohm beyerdynamics, etc); can only buy through massdrop
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.  
TL:DR: 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.
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
Great review.  Really liked your descriptions of the filters.  Thanks!
Great Review! Now I'm actually considering this piece of equipment.
Eh are you sure they can't drive the HE6's well?
Like, I've heard they make the K7xx's scale amazingly, and are a perfect match for the HE400i, but there are also claims that it can drive the LCD4's with no shortcomings, and the HD800's.
If they can't drive the HE6's should I just avoid them? (I own a pair and my Valhalla kind of got knocked off my counter by my dog)


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