Fiio K9 Pro Review

Fiio Main

It’s difficult to find a solid headphone amp for less than a thousand that has all of the features you need for your HiFi set up. You have certain selections from iFi like the ZEN series, or even portable devices that can technically be used for your desk like the IDSD Signature. However, there must be more out there to choose from in that price range, but in reality the competition is more sparse than desired. Fiio has now come out with a new desktop amplifier for $699 that looks like it can do a little bit of everything. Does it answer the call for more mid priced amps?

Fiio Items

What You Get

  • K9 Pro amplifier *1
  • Power cord*1
  • Square port USB data cable*1
  • Bluetooth antenna *1
  • Headphone jack adapter*1
  • Quick start*1 (7) Card*1
  • Instruction card for voltage selection*1
  • Fuse*1

Fiio headphone connectors


It’s not everyday that we see an amplifier with this type of design occupying this price range. You’d usually need to spend a bit more than a thousand to get a device with a huge black chassis like this one. The F9 is presented like a true high-grade amplifier, with a ridged and bulky look, but also sleek and stylish with a durable build that’s still and impressive for its price. The face of the K9 is very clean, evenly spacing its many inputs, switches, and LED indicators in a way that clear and easy to immediately recognize. However, if you’re me, then you might have completely missed the 4-pin XLR headphone connector hidden underneath a cap on the left hand side of the front panel that you thought was a button and was confused about what it did.

Once you figure out that this is in fact, not a button but a dust cap for the XLR input you also might see the quarter-inch and 4.4mm headphone connectors underneath. Let’s talk next about the K9’s big beautiful volume knob. The silver outline and LED backlight really makes this design exceptionally modern and polished. This volume pot also has a great feel to it, giving you proper weight when turning the dial. One the right hand side of the K9 you have a selection of switches and buttons that operate output, gain, input, and power.

Fiio Rear

For outputs you can switch between DAC, pre-amp, and HP. This is perfect for when you’re using the K9 outside of just strictly for headphone use, or when you’re connecting to outboard gear. The K9’s gain options range from -3dB for low, 6dB for medium, and 16dB for high. This only usable when connected through the amps quarter-inch jack. Next to your gain switches are your input selections, which are indicated by small LED lights just above this row of switches. You can choose from USB, OPT,COAX, LINE, and Bluetooth.

The rear panel of the K9 is host to a ton of connections. Here you can connect your power supply, USB, coaxial, and optical inputs, as well as 4.4mm balanced and 3.5mm single ended line in and line out.

Fiio THX


Inside of this big black box are some significant components that shape the K9’s signal flow and sound signature. One of the most important elements to consider is the return of the legendary AK4499 chipset capable of 384kHz and native DSD256 decoding. Adding onto the K9’s rare mechanisms is its THX certified AAA technology. With its use of 788+ chips, the K9 Pro is more than capable of driving high-impedance headphones with output up to 46Vpp transients for a high level of efficiency. Don’t forget that the K9 also has high-res Bluetooth capabilities, supported by a Qualcomm QCC5124 chip that can handle CODECs like LDAC aptX HD and aptX Adaptive. Using the K9 Wirelessly is simple and shouldn’t give you a hard time with drop outs. It can operate at high bitrates and low latency making this a great companion for HiFi gaming setups. If you’re worried about MQA, the K9 is a proper decoder for listening to high-resolution tracks using Tidal.

Sound Impressions

There are many ways to break down the K9 in terms of sonic fidelity, but for my experience, it really came down to how effective that THX AAA amplifier is. I spent my listening mainly using the AK99 DAC with Apple Music lossless and a few different over-ear headphones. One of its standout qualities that I found most common in between each headphone I tested was the amps ability to produce incredibly smooth signal flow.

I tried listening to “The Cursed Clock” by Nine Inch Nails, which purposefully consists of very bright piano tones and a piecing ringing effect cutting through the track. Using the K9 with the Final Audio D8000 significantly reduced the harshness of this effect while keeping its prominence and sonic fidelity within the mix. No impression of the musicality is lost, but the timbre is also much easier to take in due to the more effortless flow the K9 provides. The other regions of the sound signature were exceptionally clean as well.

Bass and midrange resonance displays an equal amount of transparency, forming a tight resolution that perfectly replicates musicality. I didn’t find the K9 to produce much in the way of added texture, instead it revealed details in a more realistic manner. In terms of gain, the K9 leaves room for improvements. I used the amp mainly at its high gain function, and I still had to turn the knob to almost 3 o’clock in order to find a comfortable loudness. This still leaves you a good amount of headroom, and headphones with lower sensitivities than the Final Audio D8000 and the HiFIMAN Arya should give you even more room for potential volume.

For the imaging and soundstage, I’ve never come across an amp in this price range that preformed such effective spatial properties. I felt like the K9 just had an unlimited amount of breathability, giving airier headphones an unrivaled presence, with the D8000 appearing the most like natural speakers. Greater immersion in the sound field is easily obtained with the K9, as it effortlessly expands upon the added dimension of open-back headphones.


For its price, the Fiio K9 is incredibly impressive with its build, input/output options, and of course its sound quality. It’s a quite large system, but it preserves a level of solidity that puts it in a class well beyond what you think a device like this should go for. We don’t have a lot of meaty amps like this in this price range, but Fiio has delivered an exceptional unit here that makes the market that much more competitive.

The Fiio K9 Pro is available at Audio46.

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Alex S. is a sound designer and voice-over artist who has worked in film, commercials, and podcasts. He loves horror movies and emo music.