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Yes, it’s big and heavy for a product that thinks it should live on your desktop. But the FiiO K9 Pro ESS’s functionality and sonic expertise is such that you might, if necessary, buy a bigger desk…


  • Speedy, deft and endlessly informative sound
  • Great standard of build and finish
  • Extensive specification, wide functionality
  • Good control app


  • Lacking in bass power and presence
  • Quite big and heavy

Key Features

  • BluetoothBluetooth 5.0 with SBC, AAC, aptX HD and LDAC compatibility
  • ConnectionsBalanced and unbalanced inputs and outputs
  • File supportUp to 32-bit/384kHz and DSD256


In well under 20 years, FiiO has gone from ‘that brand that makes affordable digital audio players’ to ‘that brand with a stack of credible audio products of many types and many prices’. 

So it’s not all that surprising that it’s ready to go toe-to-toe with some of the acknowledged market leaders – and at serious price-points, too. The K9 Pro ESS desktop headphone amp/DAC is just the latest demonstration of FiiO’s ambition…


The FiiO K9 Pro ESS is on sale, and in the United Kingdom it’s yours for £699 or thereabouts. You’re looking at $849 or so in the United States, and it will set you back AU$1349 in Australia. 

There are plenty of desktop headphone amp/DAC combos that cost far less than this, of course, but equally it’s possible to pay quite a lot more. And even at more-or-less the same money as this FiiO, you have products from the likes of Cambridge Audio, Chord and iFi to choose from. 

So the K9 Pro ESS has to do a little more than just show up with some ill-advised gold trim if it’s going to make an impression…


  • 70 x 200 x 220mm, HxWxD
  • 2.75kg
  • Physical and app control

Yes, that gold trim. It seems understandable that FiiO, having designed a purely functional aluminium box in which to contain a lot of complicated electronics, might want to give it a bit of visual pizazz. 

But while I’m aware that different people have different tastes, the application of shiny gold to the outside of the volume control and the ‘Pro’ badge on the fascia looks – to me, at least – a bit tacky. You, of course, may feel differently – although that’s before I mention the ring of light surrounding the volume control. It changes colour depending on the quality of the incoming signal. 

FiiO K9 Pro ESS stand
Image Credit (Trusted Reviews)

Bling aside, the K9 Pro ESS is a hefty, well made and entirely unremarkable-looking device. Up front, there are balanced and unbalanced headphone outputs – 6.3mm, 4.4mm and 4-pin XLR. On the side – and where the side is all depends on how you orientate your K9 Pro ESS. FiiO provides a little stand in the packaging so it can stand upright as well as lie down – there’s a USB-C socket. 

But on the rear panel, of course, is where the bulk of the action is – a Bluetooth aerial, a USB-B socket that can handle incoming signals of up to 32-bit/384kHz standard, a digital optical (24-bit/96kHz) and digital coaxial (24-bit/192kHz) make up the digital inputs. In addition, there are a pair of line-level stereo RCA inputs, a corresponding pair of outputs, a 4.4mm balanced input and a pair of 3-pin balanced XLR outputs.

FiiO K9 Pro ESS connections
Image Credit (Trusted Reviews)

The fascia features some physical controls beyond the flashy volume control. There is a switch to select your output option (choose between DAC, preamp and headphones), another to set the appropriate gain level (high, medium or low) and a button that can either mute/unmute or wake/standby. A control to cycle through your input options (and to move the corresponding little LED) is the final control.    

FiiO K9 Pro ESS inputs
Image Credit (Trusted Reviews)


  • THX AAA 738+ amplification
  • ES9038PRO digital-to-analogue conversion
  • Bluetooth 5.0 with SBC, AAC, aptX HD, aptX Adaptive and LDAC compatibility

As befits a quite expensive desktop headphone amp/DAC, the FiiO K9 Pro ESS is extensively specified. And then specified some more.

The fundamental layout of the K9 Pro ESS is balanced, with entirely separate power supplies for the digital and analogue sections. A notably large linear power supply (with a low-frequency transformer and a quartet of big capacitors) delivers the goods. The PCB is organised to keep the analogue signal loop separate, and audio signals isolated from the power supply too.

A pair of ES9308PRO DAC chips take care of the digital-to-analogue business – there’s one for each of the stereo channels. And then the signal is amplified by a couple of THX AAA 738+ chips that are potent enough to mean that even the most truculent headphones can be efficiently driven without problems. 

Wireless stuff is handled by a Qualcomm QCC5124 chipset. Streaming is via Bluetooth 5.0, and there’s compatibility with SBC, AAC, aptX HD, aptX Low Latency, aptX Adaptive and LDAC codecs. Which should, by any reasonable measure, be plenty. 

FiiO K9 Pro ESS app

There’s also the FiiO Control App available for use. It’s a useful, if fairly brief, app – but it’s very useful indeed for adjusting EQs, selecting between different filters, confirming the standard of the digital audio file the K9 Pro ESS is dealing with, checking for firmware updates and what-have-you. It’s stable and logical, which is about as much as you can realistically ask for. 

Sound Quality

  • Fast, accurate and involving sound
  • Great detail retrieval
  • Could conceivably sound punchier

No matter if you use the FiiO K9 Pro ESS as a desktop DAC and headphone amplifier, or as a full-system DAC or preamp, the fundamental priorities of its sound don’t really change. You can chuck everything from Public Enemy & Anthrax’s Bring Tha Noize to Taylor Swift & Ed Sheeran’s Everything Has Changed at it, and it’s unflappable. In any and every circumstance, the FiiO is a fountain of information.

Its facility with detail retrieval is almost uncanny. No element of a recording is too minor, too far to the back or the side of the soundstage, or too transient to elude the K9 Pro ESS. If it’s present in a recording, the FiiO will tease it out – and then put it in its proper place, without overstating it or coming on like some laboratory instrument. It’s a spirited and entertaining listen, and not too many products at any price manage to combine that trait with the powers of analysis the K9 Pro ESS demonstrates.

FiiO K9 Pro ESS vertical orientation
Image Credit (Trusted Reviews)

And while this is of benefit throughout the frequency range, it’s the midrange that benefits most of all. The FiiO’s facility with vocalists is tremendous – you’re never in any doubt as to a singer’s motivation, their emotional state, their character, or their attitude when the K9 Pro ESS gets hold of them. And the fact that the soundstage the FiiO creates is so generously proportioned and so rigorously organised means that singers get all the space they need in order to do their thing – even if there’s a sonic maelstrom happening at the same time.   

The top of the frequency range shares the same balanced and neutral tonality as the midrange, and has just as much going for it in terms of accuracy and insight. The level of attack is nicely judged, and there’s enough bite and substance to treble sounds to prevent them getting overly hard or splashy.

It’s at the bottom of the frequency range that the FiiO is just a little less positive and commanding. There’s no shortage of extension, and the same authority where attack and decay is concerned is apparent too. Detail levels are high, and there’s more than enough variation where tone and texture are concerned to make the small dynamic differences in bass sounds obvious. 

FiiO K9 Pro ESS dial
Image Credit (Trusted Reviews)

There’s just not as much body to the low end as is ideal – the K9 Pro ESS can’t summon quite the thrust or power required to make the bass properly thump like you know it should. And so far all its speed and dexterity, and for all that the control of the low end means rhythmic positivity is excellent, ultimately the FiiO just isn’t all that hip-shaking a listen. You can mitigate this trait just a little by using headphones that are on the chunky side where low frequencies are concerned, but you’ll struggle to eradicate it.

Switch to using the K9 Pro ESS as a Bluetooth receiver and the finest of details start to go astray. The soundstage is nibbled at, too – it’s no longer quite so expansive nor so martially organised. This is all relative, though – by prevailing standards the FiiO is an insightful and full-scale listen. Even if there’s every chance you’ll hanker after a little more bass than is forthcoming. 

Latest deals

Should you buy it?

You know that knowledge is power: What this DAC/headphone amp doesn’t know about revealing the finest details of a recording isn’t worth knowing.

You’re all ‘bout that bass: Some people prefer whippets to bulldogs, after all – and there’s no denying the FiiO is as lean and rapid as a racer, rather than as musclebound as a fighter.

Final Thoughts

At first, I thought the K9 Pro ESS meant FiiO had mistaken ‘physical weight and size’ for ‘expensive and high-performance’ – it wouldn’t be the first company to have confused the two, after all. But although the K9 Pro ESS isn’t perfect (its sound is a touch lightweight and its physical presence is quite the opposite), it’s got an awful lot to recommend it – the point I’d consider tidying my desk to make space for it.

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How we test

We test every DAC/headphone amp we review thoroughly over an extended period of time. We use industry standard tests to compare features properly. We’ll always tell you what we find. We never, ever, accept money to review a product.

Find out more about how we test in our ethics policy.

Tested for more than a week

Tested with real world use


What is the difference between K9 Pro and K9 Pro ESS?

The difference between the K9 Pro and ESS models is that its successor uses a ESS Sabre chipset DAC, hence the ‘ESS’ in the model name.

Full specs

Size (Dimensions)
Release Date
Model Number
Frequency Range

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