Setup, Customization, and Performance

Desktop Setup

Given the FiiO K9 is a DAC/headphone amp/preamp all-in-one unit, the number of wires that go in the back is fully dependent on how you plan on using it. I suspect most readers of this review will be using it primarily as a USB DAC for input, although the other inputs can be quite handy with non-desktop sources, such as a CD player, a phono stage, or even a TV. I ended up using the USB DAC functionality paired with my desktop PC which means using the provided USB cable—sadly not Type-C as previously pointed out—in addition to the power cable/external power supply itself. The attached Bluetooth antenna means you can also easily pair this with your phone, for example, and you can also use a longer power cable if the default one does not work out well. The stand accessory provided in the box enables using the FiiO K9 in a vertical orientation if you are short on desk room. It also naturally angles the inputs and volume knob for easier access as seen above.

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Turning on the FiiO K9 is as simple as flipping the switch on the back. You will hear an audible click and notice the USB input indicator LED turn on along with a color flow effect on the LED ring surrounding the volume wheel itself. This seems to have been borrowed from the FiiO K7 albeit now we do not have a cover and neither is there a firmware to turn off the LEDs if this is not to your preference. On the other hand, the LEDs are effectively indicators here to signal the audio playback sampling rate and/or Bluetooth codec in use. So it's a case of function and form alike and I'd say it's not distracting by any means. Interestingly, despite the DAC and USB encoder both allowing MQA playback, the FiiO K9 does not support it. Not that it's a big deal anymore with MQA under administration owing to a declared bankruptcy!

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Using the FiiO K9 with Windows 10 or 11 doesn't require any additional drivers, with Windows recognizing the device automatically. We see that the default firmware provides PCM playback options up to 32-bit, 384 kHz in addition to DSD256. The USB decoder comes in handy here, and all your favorite players will recognize the device in WASAPI (Windows Audio Session API) mode accordingly. There should be equivalent drivers pre-configured in newer macOS builds too, but I am not familiar enough to speak on this front.


If you have a device that doesn't accept UAC 2.0, or even if you simply want to try other options, then the FiiO USB audio device drivers will be of interest to you. You can find the installer on this page and the latest version at the time of testing was v5.50.0. Note that FiiO suggests using the older v5.30.0 drivers but they weren't recognizing the K9 in the control panel for whatever reason. Use the newer drivers anyway, the K9 is clearly supported there. Installation is trivial as seen above, and these drivers now allow you to make the most of the FiiO K9 within the Windows operating system.



Installing the drivers creates a new output device in Windows as well as provides access to the FiiO USB Audio Control Panel. Just be sure to set the format to your desired level and choose the ASIO (Audio Stream Input/Output) buffer size that suits your latency requirements. With that done, go to your media player of choice. I demonstrated above with JRiver again and set the audio player to the same output. At this point, you can implement further tweaks, but those are player dependent. These drivers also provide 24-bit playback support for those who find it useful. The drivers are also recognized by Roon, which is quite possibly the best thing you could have if you tend to go with local storage of music files over streaming. I have more or less replaced JRiver with Roon now for its various features and, If you are interested to give it a try, you can use this link to get a free 30-day trial (I don't get anything out of it).



The FiiO K9 is very easy to pair with your phone or other Bluetooth sources too with the Qualcomm SoC providing decent codec support in the form of AAC/SBC/aptX/aptX LL/aptX HD/aptX Adaptive/LDAC. It is also supported by the FiiO Control mobile app available for both Android and iOS. I've used it in conjunction with the FiiO Music app on my phone for a good user experience. Adding the K9 is simple enough given the app detects it immediately and then you get further controls over the UAC, having a useful equalizer—unfortunately only available in Bluetooth input mode—as well as other features such as selecting the low pass filter, input source, Bluetooth codec, and even updating the device firmware. I will however complain to FiiO here since it claims to provide PEQ (parametric EQ) but really this is just a graphical EQ since we don't have any control over the frequency or Q-factor outside of the set 10-band EQ and the various presets on offer—this is not PEQ and claiming that the K9 supports it is misleading to the end user.

Audio Performance

Now that we have gone through the capabilities and set up of the FiiO K9, let's talk more about its capabilities in terms of driving headphones. The K9 has an output impedance <1 Ω, which makes it suitable for pretty much everything including ultra-low impedance IEMs, and it can provide ~1.5 W @32 Ω and ~200 mW @300 Ω off the 1/4" jack, which goes up to ~2 W @32 Ω and ~780 mW @300 Ω on the balanced outputs that will no doubt be the way to go with higher impedance headphones such as the FiiO FT3 or lower sensitivity planar magnetic headphones too. I am also glad that the default low gain mode allows for using the FiiO K9 with IEMs much better than I was able to with the FiiO K7 before, and this is one of the stronger reasons to justify the K9 over the less expensive K7 in my books. As per usual, you get a decent range of volume control here and only bump up the gain levels as needed. The background is plenty dark enough to where it should not by itself be holding back the vast majority of head-fi transducers on the market.


I spent a couple of weeks using the FiiO K9 with a variety of headphones and IEMs to fully test how it sounds, knowing already that it goes low enough to not have sensitive IEMs hiss and also has enough juice for demanding headphones too. The line out features worked very well and I have no complaints on that front having used this with the FiiO SP3 speakers which we will discuss separately soon enough. I did not notice any coloring of the sound signature, and measured it to be sure. Indeed, this was the flattest measuring FiiO source I've tested to date. It seems clear that the company has a mature platform on its hand, having used a similar design architecture with the older K9 and K9 Pro ESS too. The DACs may be different and there may be fewer inputs but it makes for a neutral yet engaging sound. FiiO is the only company I know which has managed to get the THX AAA 788+ amp to not sound completely dead, and the same continues here with the K9. If anything, having the Q7, M15s, K9 Pro ESS, and the K9 all together made it easy to conclude—for me anyway—that the less expensive DACs used here do not influence the sound signature as much as the power delivery stage itself. Here the K9 does enough to please those who care less about source measurements and more about chain synergy too. For added context, the TOPPING E50/L50 stack comes off more neutral, almost bright by comparison, whereas the YULONG Aurora with its Class A buffer arguably sounds better with vocals but has some weird design choices and doesn't offer as much weight to bass notes.