This will be a short review as there are a few other reviews that go into great depth listing all of the specs. I mainly joined this tour to basically get another chance at the X7 and try the amp modules. When I first joined the tour for the X7 when it first came out I did not have a great experience as the UI still was not stable. Now, the UI seems a lot better which also translates to a better user experience. The sound with the AM1 was ok it was a bit sterile with my setup and did not really compare the two. I used the AM2 the majority of these tour and absolutely fell in love with the sound. I really could not put this down and used it a lot. I mainly was comparing this to my Onkyo DP-X1, I downloaded Tidal HIFI and loaded all of my songs on to the player mainly using my Alclair RSM CIEM.
The X7/AM2 is a bit more musical then the DP-X1 and is easily enjoyable for hours on in. There is great bass and clarity at the same time never overpower nor bleeding into the highs. I actually considered buying this with the AM2 skipping buying the AM1 as that sound sig was not to my liking. What more is there to say, great buy to change the X7 for the better.
 
The X7 with AM2 is a great buy, I wish it had 2 mSD slots (wishful thinking for the X7-2) as you can never have to much memory. 

Table of Contents

• Introduction
• Specs
• Build Quality
• Battery Life
• Sound quality
  • Volume-matched Comparisons with other amp modules
• Conclusion

 
(Before I even begin with the introduction, I wanted to say that the above table of contents is for your convenience. I’ll also include a tl;dr summary at the beginning of each major section.)
 
Introduction
 
Tl;dr: FiiO lent me the unit for my honest opinion, and a bit of background about myself.
 
 
This review is specifically about the AM2 amp module designed for the FiiO X7 DAP. For a review of that particular player, you may want to click here for my review or take a look at the many other reviews of the X7.
 
I would like to thank FiiO for letting me demo all of the X7 amp modules in exchange for my honest opinion!
 
 
 
Specs
 
Before we go onto the rest of the review, let’s post some specs for comparison, shall we?
 
 
Build Quality
 
Tl;dr: Great build quality. Not much to complain about, except perhaps the color doesn’t exactly match the rest of the X7.
 
The AM2 amp module continues the tradition of great build quality as found in the rest of the X7. The metal feels smooth and polished, with no rough edges. AM2 snaps in easily and is flush to the rest of the X7’s body, where it remains tight and secure once you put in the included screws.
 
It pretty much looks like it belongs with the rest of the player, if it weren’t for the fact that it’s a much darker shade of grey compared to the rest of the X7’s brighter silver color. Honestly, this isn’t such a big deal for me since I’m not looking at the player for much of the time, but it is a bit weird to see three different shades of grey on the back of the player when AM2 is attached (one at the top, one in the middle, and AM2 on the bottom). There’s also the fact that you won’t notice this at all if you buy a cover for the X7.
 
Pretty much no complaints from me here.
 
 
 
Battery Life
 
Tl;dr: You take a bit of a hit to battery life compared to AM1, but to me it’s a good compromise between driving power and power consumption.
 
Unfortunately, more power on tap means that the battery life will inevitably suffer. However, in the AM2’s case this isn’t as bad as you might think it to be.
 
I conducted my battery life testing under the following conditions: the X7 powering the Etymotic ER4S at low gain at a volume level of 39, using the FiiO Music app in Android mode, and with the screen, pulsar light, Wi-Fi, and Bluetooth off. The music the X7 cycled through was mixture of CD-quality and high-res FLAC, as well as some DSD tracks. CD-quality FLAC tracks made up the vast majority of the music.
 
Under those constraints, as seen in the screenshot below I managed to get 7 hours and 41 minutes of playtime. In comparison, with the AM1 amp module volume-matched and under the same conditions, I got 8 hours and 22 minutes of playtime. With the AM2 amp module under the same conditions and volume-matched, the X7 could play for 7 hours and 41 minutes. With AM3 SE powering the same headphones volume-matched and playing under the same conditions, the X7 could squeeze out 7 hours and 14 minutes out of its battery.
 
In my opinion, this isn’t bad at all, considering I get decently more power to drive headphones but lose less than an hour of battery life. However, there’s no doubt that if you want the most battery life at all costs, get the AM1.
 
 
 
Sound Quality
 
Tl;dr:  The AM2 does very well with IEMs, and decently with moderately power-hungry full-size headphones. It’s a clear step up from AM1, however as expected it does lose out to AM5 and AM3 balanced.
 
Headphones primarily tested with: FiiO EX1/Dunu Titan 1, Etymotic ER4S, Hifiman HE-400i, and Sennheiser HD700.
 
Ok, enough with the build quality and battery life. How does it sound, you might ask?
 
I would say that the AM2 has a mostly neutral tone, perhaps a bit “relaxed” or some might call that a bit “warm.” Despite this overall character, AM2 is still quite detailed and has pretty good separation. This, combined with the moderately wide but with good depth soundstage, means that the AM2 provides some very good imaging (horizontal sounds) and layering (easily perceiving how close or far away sounds are). Another way to say all of this is that AM2 presents a convincing and coherent 3D soundstage.
 
AM2 also has enough power and finesse to drive almost all IEMs and even enough power to drive moderately power-hungry full-size headphones adequately, if not perfectly. For example, the AM2 drove both the FiiO EX1 (16 ohms) and the Etymotic ER4S (100 ohms) without hiss and without lacking power. The AM2 also powered my Hifiman HE-400i and Sennheiser HD700 pretty well, though by comparison to the AM5 amp module the AM2 did lack a bit here.
 
Volume-Matched Comparisons
 
All comparisons here were conducted under volume-matching with a C-weighted SPL meter.
 
Vs. AM1
 
The AM1 in comparison to the AM2 has a slightly wider soundstage. It also sounds “brighter” and a little more “metallic” and “brittle.” The AM2, in contrast, sounds somewhat “warmer” but also more natural in that notes are still as detailed without sounding like the detail is forced. This also means that stuff like vocals are given more “depth” and nuance on AM2. AM2’s soundstage is also very slightly narrower but also slightly deeper, making for a more 3D soundstage in which it is easier to perceive depth and layering of notes.
 
Separation and bass response sound about the same to me on AM1 and AM2. Perhaps some might say that the AM2 sounds “bassier” because of its more relaxed mids and treble.
 
All of the aforementioned characteristics are audible even on something as easy to drive as the FiiO EX1 IEM. The extra power also makes it so that the bass is more prominent and notes are less strained and grainy on full-size headphones being powered by the AM2 compared to the AM1. Overall, I think the AM2 sounds better than the AM1 no matter what headphone or IEM you are using.
 
Vs. AM5
 
AM2 and AM5 for the most part sound very, very similar. However, notes seemed to be a bit clearer, perhaps sharpened up a bit (but not overdone) on AM5. This can, for example, make wind instruments seem a tiny bit airier and vocals a bit “breathier.” AM5 also perhaps has a slightly wider soundstage, but depth of the soundstage is comparable between the two. I must emphasize that all of these differences are relatively minor.
 
To be honest, there’s not much improvement (if at all) in going to AM5 over AM2 for IEMs, even for harder-to-drive ones such as the ER4S. However, there seems to be an audible, if not major, improvement in using AM5 over AM2 to drive full-size headphones – for the HD700 and HE-400i, I thought I heard on AM5 that notes were slightly less grainy, slightly more open-sounding, that there was slightly more hard-hitting bass, and with slightly more separation.
 
So AM2 or AM5? Honestly, I think the biggest difference between the two is really how much power each can provide, and so this really depends on what type of headphones you listen to. If you mostly listen to IEMs with perhaps the occasional moderately power-hungry full-size headphone, go for AM2 because it has better battery life. If you frequently listen to hard-to-drive full size headphones, go with AM5.
 
Vs. AM3
 
First off, overall AM2 sounds very similar to AM3 in terms of soundstage size, detail retrieval, separation, imaging, etc. Perhaps AM3 is very slightly “sharper” and clearer sounding than AM2, but honestly I could just be imagining it.
 
As for AM3 SE vs. AM2, the AM2 is as capable as AM3 SE in powering IEMs (even more power-hungry ones). I honestly can’t tell much of a difference here.
 
However, for full-size headphones, AM2 actually comes out ahead. AM2, to me, simply produces less grainy notes and better-controlled and better-textured bass than AM3 SE when powering larger headphones.
 
But with AM3 balanced, the tables completely turn. (At least with full-size headphones, since I don’t have any balanced IEMs to test with.) Notes sound more airy and less constrained than on AM2. Bass is also deeper, more well-textured, and better-controlled. AM3 balanced’s soundstage is also slightly wider and deeper, and separation on AM3 bal. is also noticeably better (which also leads to better imaging and layering/depth perception).
 
So if you are planning to just use single-ended headphones, go with AM2 since it sounds just as good or better (depending on the headphone) with better battery life. But if you want to get the most out of your larger headphones, go with AM3 balanced.
 
 
 
Conclusion
 
Tl;dr: The first and last sentence of the next paragraph.
 
At only $99, the AM2 amp module is a relatively affordable way to not only give your X7 a bit more power to drive a wider range of headphones, but also a boost in overall sound quality. If you really want the most out of battery life for the X7 and mostly use IEMs, get the X7 with the AM1 amp module as it provides the best battery life and it does still sound pretty decent. However, if you don’t mind somewhat lower battery life, get the AM2 as I do think it gives a noticeable increase in sound quality no matter what headphone you use. The AM2 very well may be the most suitable amp module for most people, as to me it gives the best combination of battery life and driving power.
 
Thanks for reading this (somewhat) long review of the AM2 amp module!

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I would like to Thank FiiO for providing me with a review sample in exchange for my honest opinion.
 
I haven't seen AM2 being featured yet on FiiO website, and only found a link to http://www.bhphotovideo.com/c/product/1232533-REG/fiio_am2_amplifier_for_x7.html where you can pre-order it.

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This is a short review of FiiO AM2 amplifier module intended for use with their flagship X7 DAP.  Paul (@Brooko) already posted a comprehensive review HERE which I highly recommend for others to read.  Due to an overload with all the reviews in my current queue - I made a decision to write a shorter impression sharing my thoughts about using this replacement amplifier module.
 
One of the benefits of X7 modular design is being able to swap amplifier modules (AM) in order to accommodate headphones with different power requirements.  It's definitely a great idea, but at the same time I think it did hurt X7 a bit during its introduction with a default IEM (AM1) module.  A lot of people judge the product based on the default configuration without realizing that upgrade can scale up the sound quality.  Don't get me wrong, original AM1 module has enough juice to drive most of the IEMs/CIEMs, efficient headphones, and even some not so efficient ones to a decent loudness level.  But for my personal taste I found low gain setting of AM1 to lack the energy and the excitement even with my IEMs.  Purely subjective opinion, but I always had AM1 on high gain, because I liked the improvement in the tonality, the texture, and the dynamics of the sound it brought to the table.
 
Using AM1 was not a show stopper, but I did get excited when I learned that medium power AM2 will feature MUSE02 opamp ICs, the same one used in E12A portable amplifier which I still hold in high regard.  As a matter of fact, I even remember when testing X7 w/E12A connected to LO how great it sounded paired up together (more body and wider soundstage).  Of course, just an opamp chip itself doesn't guarantee an identical sound improvement, but MUSE02 has its own characteristic and "coloring" which I definitely like and was looking forward to hear integrated with AM2.  After all, X7 is about a portable pocket friendly design, and I personally don't want to carry it stacked up with additional external amp.  That's where the beauty of modular design comes into the picture - you just remove one amp module and replace it with another one, integrated seamlessly with a footprint of X7.
 
Unboxing & accessories.
 
I know, some of you are thinking - unboxing of an add-on accessory which comes with its own accessories?  Well, if you are familiar with FiiO products, you will know they take a lot of pride in presentation and packaging of everything they sell, regardless if it's a flagship DAP or a cable or just an add-on amp module.  AM2 arrived in a nice little box with a cool artwork image on the cover, focusing on the bottom part of the X7.  Inside of it, there is a storage tin box with a precise foam cutout where you will find AM2 module.  Presentation is first class, even for small product like this.  The tin storage box can be used for your original AM1 module, or for anything else if you remove foam insert.
 
That would have been plenty, but FiiO also included more accessories.  You get a pair of mounting T5 screws, as a spare replacement.  You don't really need it since you already have a pair holding AM1 in place, but nevertheless - in case if you drop or loose one during replacement, now you got spares.  And very thoughtfully, you get 5 theme stickers for amp module.  I have reviewed X7 sticker accessories HERE, and made a comment that if you going to apply these stickers and replace AM module later - the bottom of X7 will be naked.  FiiO thought of everything, and added all the required stickers for the amp module to match the ones you already applied (if you have).  This is a very thoughtful idea.
 
 
 

 
Design.
 
Here you will find no surprises - it's a mirror image of the original amplifier module, just a slightly different darker color shade.  Not sure what was the purpose of the color change, or maybe because my X7 unit is part of the first batch build intended for reviewers where the color was a few shades lighter.  But either way, I don't apply stickers and I use FiiO genuine case with my X7, so color difference is irrelevant to me.  I also noticed an updated print on the back, indicating this is AM2 model.
 
Of course, the main difference is inside where FiiO updated opamp from OPA1612 to MUSES02 (the same one used in E12A).  This is not a high power update, thus a reason why FiiO refers to this module as medium power, and we are still waiting for balanced output module and high power module to be released in a near future.  According to the comparison spec of AM1 vs AM2, the power increase is load dependent, approximately 1.75x more with 16 ohm load (200mW to 350mW) and up to 3x more with 300 ohm load (10mW to 30mW).  Obviously, 300 ohm is not the limitation since I was able to drive my 470 ohm R70x and Momentum 2 Wireless (in wired), both with an authority.
 
Another interesting fact, module replacement is fully plug'n'play compatible.  You don't need to power down your X7, just take off the screws, unplug the module, and carefully plug the new one in - X7 recognizes the new amp module and changes notification bar id from AM1 to AM2.  To be on a safe side, you can always power down X7 before you replace the module, but just wanted to mention that plug'n'play does work.  Also, keep in mind with a higher power module, especially when you run it in a high gain, expect a battery life penalty where I noticed a reduction on average of about one hour or less.
 
 
 
Sound analysis.
 
Instead of going through every headphone and noting the difference, I decided to summarize my finding across my usual suspects, such as R70x, PM-3, MSR7, ZEN 1.0, ES60, DN2kJ, and U12.
 
X7-AM1:
 
Low Gain – detailed sound but lacks some energy and overall not as dynamic.
 
High Gain – sound is more detailed and I hear more transparency and clarity, great low end and treble extension, excellent layering/separation, not as much body and the sound is more analytical and neutral.
 
X7-AM2:
 
Low Gain - soundstage expands with more width, there is more weight in the low end with more sub-bass; lower mids have more body but I felt like there was a bit of a bleed from the bass which made sound warmer and even a little more congested.
 
High Gain - the same soundstage expansion, low end is a bit tighter and feels more controlled, lower mids still have more body but the sound also has more clarity, more dynamics, even some slight improvement in layering/separation.  To my ears, in high gain the sound is not as congested as it was in low gain.
 
Comparison to K5 and E12A.
 

 
 
X7-AM2 vs X7 w/E12A - nearly identical sound signature and tonality, similar soundstage expansion except w/AM2 I hear it a bit wider.  The main noticeable difference to me is in X7 w/AM2 bass is faster, tighter, and a little more articulate, while w/E12A it is slower and not as tight.
 
X7-AM2 vs X7 w/K5 - w/K5 soundstage is noticeably wider, while depth/height is the same. Everything else, including speed and tonality, is the same.  Of course, K5 can go up in power to drive even the most demanding headphones, thus X7-AM2 with K5 is one heck of a combo.
 
Conclusion.
 
I know a lot of X7 owners will have a question if they should upgrade to AM2 module or stay with AM1.  It’s an easy decision if you feel like you’re running out of juice with X7 and don’t feel like carrying your DAP strapped to an external amp.  AM2 can give you that extra boost, especially if you have higher impedance cans.  But if you are listening to more efficient headphones on the go and higher impedance full size at home – perhaps it makes more sense to invest into K5 docking amp which cost almost the same as AM2 module.
 
The question becomes a bit more challenging to answer if you are satisfied with AM1, similarly to my situation where I was happy using it in high gain.  Extra power aside, it all comes down to a personal sound preference.  I find AM1 to sound a little thinner and colder and more analytical which pairs up fantastic with neutral and warmer headphones, though no slouch with bright analytical headphones as well.  Stepping up to AM2 will give sound more body, more weight, improves low end control, and to my ears makes soundstage a little wider – not too much but just enough to notice.  AM2 improvement brought the sound closer to K5, though K5 sound is still wider.  Thus, you have to decide if you want to keep your sound improvement portable and go with AM2 ($99) or compliment your X7-AM1 with a desktop K5 docking amp ($109).

For larger (1200 x 800) images, click any picture

INTRODUCTION

This is going to be a pretty short review by my standards (do I hear you letting a sigh of relief?).  I reviewed FiiO's TOTL Android based touch screen DAP – the X7 – in early November. At the time I gave it a 4/5 (80%), mainly due to the early firmware version, and some missing features in the UI. If I was to review the X7 today, my only con, or issue, would be the battery life.  I’ve got used to the size, and the latest updates (now at firmware version 1.8) have improved the X7 to the point that for my uses, it is practically the complete TOTL DAP I’d always thought it was going to be.
 
I can now go artist, album, track, the DAC works beautifully, the blue light can be turned off, the battery indicator seems to be a lot more accurate, and with the release of the AM2 medium power module, those with harder to drive cans now have some of the power they were looking for.
 
ABOUT FIIO
By now, most Head-Fi members should know about the FiiO Electronics Company.  If you don’t, here’s a very short summary.  FiiO was first founded in 2007.  Their first offerings were some extremely low cost portable amplifiers – which were sometimes critiqued by some seasoned Head-Fiers as being low budget “toys”.  But FiiO has spent a lot of time with the community here, and continued to listen to their potential buyers, adopt our ideas, and grow their product range.  They debuted their first DAP (the X3) in 2013, and despite some early hiccups with developing the UI, have worked with their customer base to continually develop the firmware for a better user experience. The X3 was followed by the X5, X1, X3 2^nd Gen (X3ii), X5 2^nd Gen (X5ii), M3 and X7.
 
FiiO’s products have followed a very simple formula since 2007 – affordable, stylish, well built, functional, measuring well, and most importantly sounding good.
 

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DISCLAIMER
The X7 and add on AM2 module were provided to me gratis as a review samples.  I have made it clear to FiiO that I still regard any product they send me as their sole property and available for return any time at their request. But I thank them for the ability to continue use of the X7 – both for follow up comparisons and also for my own personal use. It is my intention to purchase the X7 from FiiO as soon as I can afford it
 
 
I have continued to use X7 and its modules for follow up reviews, and I recently inquired if I could purchase the devices from FiiO.  They have insisted I keep the X7 + modules for my own use. So I acknowledge now that the X7 I have is supplied and gifted completely free of any charge or obligation.  I thank FiiO for their generosity. 

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PREAMBLE - 'ABOUT ME'.
 
 
REGARDING THE X7
This review is essentially about the AM2 medium power amp module released by FiiO for the X7.  For a detailed look at the features of the X7, and a quick run-down on the AM1 (default) IEM module, I would recommend you read my X7 review or indeed any of the 30 something reviews on the X7 currently listed.
 
This is a purely subjective review of the AM2 module – my gear, my ears, and my experience.  Please take it all with a grain of salt - especially if it does not match your own experience.
 

THE REVIEW

PACKAGING AND ACCESSORIES
The AM2 arrived in a small black retail box measuring approximately 90 x 120 x 25mm. On the front of the sleeve is a picture of the bottom half of the X7 with AM2 module attached and some text telling you that this is the AM2 amplifier module.  I’m actually surprised it doesn’t mention the X7 specifically though. On the rear of the box are QR codes which will take you to FiiO’s website or Facebook page.
 
 
Removing the box reveals a plain tin box with a nice powder coated finish. Removing the lid reveals a black cardboard envelope, and under this is a foam cut-out with the AM2 module nestled safely inside.
 
Inside the envelope is a warranty booklet in multiple languages, a full set of stickers (which match the ones from the X7) and 2 replacement screws. I was really surprised to see the stickers, but this is FiiO definitely thinking out of the box.  If you’ve brought and applied stickers to your X7 already, the last thing you’d want is a new amp module with no adornments. Although I don’t use them – this is a nice touch.
 
As far as the AM2 goes – the other nice thing to note is the rubber dust cover/protector over the connection pins.
 
The one thing I’m surprised at that is missing though is the specifications – FiiO is usually relay good at including these.  Now it could simply be that they were omitted because this is a review sample – but worth noting anyway. The good thing is that as per usual, FiiO have already listed the spec for the AM2 in the X7 section on their website.
 
TECHNICAL SPECIFICATIONS
The table below lists most of the relevant specifications.  I have (as a comparison) also listed specifications from the default AM1 module.  To give a further idea, I’ve also listed the amplifier specs from FiiO’s X5ii DAP.
 
 
BUILD / DESIGN
 
 
Not much to talk about here – the AM1 and AM2 have identical dimensions, and the main differences are internally (obviously right?), but also in their exterior colouring.  Where the AM1 has a brushed titanium appearance on the front and powdered titanium appearance on the rear, the AM2 is a slightly darker shade, and powdered finish on both front and back.  Otherwise both look and feel identical.
 
 

Replacing the modules is extremely easy – just a matter of using the small hex screwdriver included with the X7 – undoing two screws, sliding the AM1 out, and sliding the AM2 in. The fit on the AM2 is perfectly flush, and the only thing very apparent with the AM2 fitted is the change in colour.  This of course disappears when used with the cover.
 
 

DESIGN – INTERNALS
Although you can’t see them, it is probably a good idea to mention the internal electronics. Where the AM1 uses an OPA1612 for voltage and AD8397 for current, the AM2 uses the higher output combo of the Muses02 and Buf634.  Both have impressively distortion, SNR, and channel separation. The biggest difference is really the voltage output, and slightly higher output impedance with the AM2 (still impressively low 0.5 ohm though). As far as power goes, the AM2 is able to produce triple the output into 32 and 300 ohms. It also has a higher peak voltage output.
 
POWER OUTPUT – REAL WORLD
So the specs are listed above, bit what does that mean in the real world?  The obvious test was going to be with my HD600, so armed with a 1 kHz test tone, and my trusty calibrated SPL meter, I set about volume matching.  To aid quick swapping in this exercise, I simply undid the screws, and left them off – so I could easily slight amp module in place, and then quickly swap as I needed to.  The SPL meter was left in a fixed position, and comparative measures taken were (with the 300ohm HD600):

1. AM1 at 77/120 = AM2 at 67/120
2. AM1 at 86/120 = AM2 at 75/120

 
Both of the above were within 0.2 dB.  I then used those measurements playing actual music and recording maximum peaks – and again both were within 0.2 dB. 
 
I also checked (using similar methodology) the Oriveti Primacy IEMs I’m currently also working on (11 ohm, 107 dB/mW), and the AM1 at 34/120 = AM2 at 27/120.  To be honest, I was actually expecting there to be a bigger volume difference between IEM module (AM1) and medium power module (AM2), but as we’ll find out below, the change in volume isn’t the only thing different when paired with the HD600 – there is also the increase in peak voltage available.
 
One final note – with the very sensitive Primacy, I asked my lovely wife Tania to check for hiss or noise at high volume (with the X7 paused).  She could faintly discern some noise at above 80/120 – but she described it as a very faint crackle rather than real hiss.  For the record, I couldn’t hear a thing.  But at the level described, the noise would disappear altogether at the deafening volumes of music emitted if you were actually listening.  So the AM2 (like the AM1) is very, very clean – and good for lower impedance IEMs (in my test anyway).
 
BATTERY LIFE
Although FiiO) publishes their own real world tests with their modules, I also like to conduct my own.  So over two days, I first used the HD600 and AM1 set to a volume of 80/120, with my library set to repeat, and then kicked it off at the beginning of the day, and checked when I got home.  For most of the time, the screen was off – apart from maybe 6-8 times quickly checking progress.  The AM1 lasted for 9 hours and 42 minutes with 4% left (this was more than my initial testing of the X7 with AM1 – so I suspect FiiO have worked on power management with some of their firmware releases).  I had Bluetooth and Wi-Fi both turned off, and the pulsar light off as well – so this would have helped.
 
Using the same scenario with the AM2, but the volume on 70/120 (to mimic an approximate volume match), I repeated the exercise.  AT the 8 hour 49 minute mark I had 5% of battery life left, so I was actually pretty happy with performance.
 
Of course this will also be dependent on what else you have running, what headphones you are driving, and also if you are using Wi-Fi, Bluetooth, or using the screen a lot.
 
SONICS (subjective)
OK – so we’ve covered the specs, build, power and effect on battery life.  I know the bit everyone is waiting for is any sonic changes.  Before we start, all I’ll say is that my ears are probably not as sensitive as many of you, I volume match very closely, and I’m subject to the same amounts of potential placebo as all humans.  The swapping for the comparisons were as quick as I could make them to preserve auditory memory (same procedure as before – screws undone – swap units, adjust volumes to the pre-set levels, and listen). I varied between rapid swapping (portions of a track about 10-15 seconds) and longer listening periods (a full track at a time).
I used a mix of my usual test tracks - http://www.head-fi.org/a/brookos-test-tracks and concentrated mainly on tracks exposing detail, dynamic contrast, soundstage, bass quantity and vocal quality.
 
With the HD600
My first test was sound staging (Amber Rubarth – Tundra), and I really couldn’t discern any change in soundstage width or depth at all.  In fact both the AM1 and AM2 sounded excellent with this track, and the obvious difference was the amount of headroom – as it is a pretty quiet recording.
 
With the bass test I got my first inkling that there was a bit of a difference.  It was very subtle, but mid-bass impact appeared a touch cleaner and quicker with the AM2.  It was very subtle though and took a lot of time before I was sure it was there.
 
This was repeated when I briefly tried the Beyer T1 with the AM2 (it was actually a pretty nice combo).
 
Listening to vocal tracks and my go-to detail track (Sultans of Swing), the other difference was in dynamic contrast – I guess what you would all energy or vibrancy. The AM1 + HD600 was a little bit more laidback, while the AM2 seemed to have a little more engagement, just a little more life (if that makes sense).  Again – to me the differences were very subtle, and both the AM1 and AM2 were excellent with the HD600, but if I was exclusively listening with the Senn, then I would use the AM2 as default.
 
With the Oriveti Primacy
I stuck with the Primacy because I’ve got to know it really well recently and if my paid work will ever slow down, I’ll be able to finally finish my review. It is a triple driver hybrid – so although a relatively easy load to drive, it can quickly show contrast particularly between bass and mid-range.
 
There was no change at all in staging, and overall tonality was virtually unchanged.  The only really small difference for me was simply in overall presentation, and this was more a feeling from listening to complete songs and switching rather than fast A/Bing. So this could merely be all in my head – but I’ll relate it anyway.  Like the HD600 – the AM1 with IEMs just seems a little more laid back / gentle / smoother overall. The AM2 is a little livelier, a little more there.  I know it’s not volume because I consistently checked throughout.
 
So would I use the AM2 with IEMs as well?  This is a tough one because both modules again sound excellent. Yes there is a bit more battery life with the AM1 and that is a big factor – but the reality is that we’re not talking hours.  For ease of use (not having to swap), I think I’ll probably leave the AM2 in place – it simply gives me options if I ever want to swap to full sized headphones.  But for those driving easy loads and primarily using IEMs, the AM1 really may be all you need. Just be aware that the changes between AM 1 and AM2 (to me anyway) are pretty subtle.
 
GRAPHS
The last thing I did before the final edit was to hook the X7 up as DAC on my PC, and record the output under loopback to see how well the overall measurements were on frequency.  Now one thing I need to stress here is the roll-off in the top end is the filter set-up on the X7 DAC section.
 
 
You’ll see the two curves for AM1 and AM2 are practically identical, so no, the Muses2 is not rolled off (unless the OPA1612 + AD8397 is too).  What it does show is an approximate 6dB difference at the same input volume (between AM1 and AM2), and overall a slow roll of about 1 dB between 3 kHz and 10 Khz.  I guess this is what gives the X7 some of its excellent “analogue” tonality. The rest of the curve is very linear. Oh and in case you are wondering, the most excellent LP% and L5 Pro both also show similar roll offs when measured as DACs – so this isn’t a fault – it’s part of why they sound so good.  

CONCLUSION

I’ve pretty much already summarised everything – but to put it in a couple of sentences …..
 
The AM2 module has a great build, is easy to fit, and sounds fantastic.  I do think there are benefits for higher impedance cans sonically, and if you are running out of volume headroom with the AM1, the AM2 will definitely help.
 
The cost in lower battery life is pretty small, and although I’ve listed it as a con – it was really expected anyway.  It’s hard to give the AM2 anything other than top marks. At an approximate release price of USD 99.00 there is nothing I can think of which makes me hesitate at all in recommending it.
 
(Edit - price adjusted, Sunny informs me the AM2 MSRP is actually $99 so even better deal!)
 
I'm expecting some people to comment on the colour difference - but as you can see, in the case it is not at all noticeable.
 
 
 
FINAL THANKS
Once again thanks to Sunny over at FiiO for giving me a chance to try the AM2 before its global release.
 
ADDENDUM
George asked me in the comments section about the roll-off I measured in the DAC section with both AM1 and AM2. I included the graphs originally because I'd had the comment that the Muses2 chip was rolled off and smoother than the OP1612 in the AM1, and I wanted to show this wasn't the case.  So tonight I reset the loopback tests, and remeasured the X7 (AM2) as well as the X3ii, X5ii, X5 original, FiiO's E17K dac/amp and also the Luxury & Precision LP5 (a TOTL player that really does sound incredible).  I'm expecting these graphs may surprise a lot of people - and especially George as he's told me he doesn't like anything rolled off - yet I know he really enjoyed both the X5 and X5ii.  The graphs were all produced with ARTA and a loopback measuring a 16/48 signal (16/44.1 with the LP5 due to driver limitations).  You'll see a very similar pattern on all the DAPs except for the E17K - and I've thrown that in there simply to show that my measuring equipment is working as intended.  These are provided simply as a matter of interest - and have no real bearing on the review.
 
 
From top to bottom, left to right: E17K, X3ii, X5, X5ii, LP5, X7