Setup, Customization, and Performance

Desktop Setup

Using the FiiO K9 Pro ESS with Windows 10 or 11 doesn't require any additional drivers, with Windows recognizing the device automatically and providing all the playback options the average end user will ever need. But of interest to audiophiles will be the Audio Stream Input/Output (ASIO) drivers, whose installer is available here. The latest version at the time of testing was 5.30.0, and these allow you to make the most of the FiiO K9 Pro ESS in wired mode. An online guide goes over the installation and setup of the generic FiiO USB DAC drivers for Windows. However, it is nowhere as involved as the mobile app coming up. Just be sure to set the format to 32-bit and choose the ASIO buffer size that suits your latency requirements—that's about it.


What it then does is create a new output device in Windows called FiiO Q series you want to set as the default before configuring the desired bitrate, and playback rate via the XMOS XUF208 USB microcontroller with asynchronous USB Audio Class 2.0 decoding. Notice the additional 24-bit options you now get; without the FiiO USB drivers, you'd be restricted to 16 and 32-bit "only." With that done, go to your media player of choice. I demonstrated above with JRiver and set the audio player to the same output. At this point, you can implement further tweaks, but those are player dependent, so I will not talk about them too much by having the photos above speak for themselves. I also deliberately choose the ASIO driver as the output to show its options, but of course went back to the FiiO Q series output to have the K9 Pro ESS running properly in USB DAC mode.

Mobile Setup

The Qualcomm QCC5124 Bluetooth SoC means the FiiO K9 Pro accepts Bluetooth input from a variety of sources, which can be quite handy for when you just want to relax with your favorite streaming platform on the phone and couldn't be bothered to plug it in. But the more impressive application of that chipset comes is the 2-way communication with the FiiO Control mobile app available for both Android and iOS. I can't speak for the iOS app, but many negative reviews on the Android platform are clear cases of user error, with several people not understanding that it is a complementary app to hardware and doesn't do much by itself. That's where the FiiO Music app comes in, but I won't talk about it here as the FiiO Control app is what you want to be using. It launches a 3-page quick start guide walking you through how to add devices and configure them using the app, and I had no issues with my phone detecting the K9 Pro ESS over Bluetooth. Note that you do not have to have the K9 Pro ESS set to Bluetooth input for this. Functionality works with all input and output options. All you have to do is make sure your app is version 2.8 or newer, and you can find the APK file for more information here.


The homepage for the K9 Pro ESS on the mobile app acts as a quick status indicator too, with a thumbnail of the device and name at the top showing it was recognized correctly, as well as the current input mode and associated playback rate. This allows for more clarity if unfamiliar with the volume knob indicator light presets, which coincidentally is the first set of options you may change. The app also allows you to switch the current input if multiple options are plugged in and available, as well as the Bluetooth codecs for K9 Pro ESS pairing—note the bad copy and paste job with the description still reading BTR3, although the rich codec support is what you should be paying attention to anyway. The settings cog doesn't have much since no firmware update with new features or bug fixes was available for the K9 Pro ESS at the time. You can change the device name if you wish, and revert to factory settings if something inadvertently broke.

Many think using an equalizer for higher-end audio devices is heresy, but I personally don't see any issue in it and even encourage it as you see fit. After all, no company will be able to completely match your needs, and finer EQ control with 5–10 points of customization is often more than a software DSP can do. FiiO does exactly that with the K9 Pro ESS by including 10-point EQ, but not with LDAC because of Sony. The Qualcomm SoC allows full EQ everywhere else, and some preset options for various music genres covering a range from 31.5 Hz to 16 kHz over 10 steps are available. There is a custom preset, but you can customize the others as well. Play around with it, especially if your headphones aren't meeting your specific needs completely, but be aware that not all headphones respond well to EQ. You may end up with increased distortion resulting in the loss of some details and audio depth.


A handy guide has been included, too. It goes over some things you may have struggled with or simply need a reminder about. I appreciate these small notes that add to the user experience, and there are more technical choices, including the seven low-pass filters on this K9 Pro ESS that pass signals on at a frequency lower than a selected cutoff frequency and attenuate signals with frequencies higher than the cutoff point. Several presets are present too, with the exact frequency response dependent on the shape of filters. Also included is a channel balance slider with a maximum range of 12 dB in either direction. I would have liked harmonic and distortion compensation options, and the DAC clock divider option of the BTR5. I suppose FiiO would rather you rely on its engineering using dual-mode femtosecond clock management for improved signal quality courtesy the 22.5792 and 24.576 MHz dual crystal oscillators for both Bluetooth and USB.

Audio Performance

This is a good segue to summarize the capabilities of the FiiO K9 Pro ESS with its dual ESS9039PRO DACs, and a way to avoid surprising anyone with the MQA support. This time around, we only get up to 8x on-device rendering as opposed to the full 16x. This is no big loss to me (get it?), but the 768 KHz PCM and DSD512 of the K5 Pro ESS with the single ES9038Q2M mobile version of the same chipset are absent. FiiO says "noise would occur" in these situations, which is why the option was turned off altogether. The maximum sample rates onboard the K9 Pro ESD thus are 32-bit/384 kHz PCM and native DSD256, which had absolutely no impact on me. While I am here, the THD+N is rated for under 0.00042% at 1 kHz/10 kΩ/dbA, with a crosstalk of ≥115dB and line level of 2.5 Vrms under the same conditions, SNR of ≥123 dBA, and noise floor of ＜1.9 μV, with these all corresponding to the single-ended line out. The specification sheets on page one have more of the relevant parameters for the SE and balanced headphone outputs, but of practical interest is the power output of each courtesy the THX AAA 788+ amplifiers. The K9 Pro ESS has an output impedance <1 Ω, which makes it suitable for pretty much everything but ultra-low impedance IEMs, and it can provide 2.1 W @32 Ω and 241 mW @300 Ω off the 1/4" jack, which goes to the same 2.1 W @32 Ω, but 1.1 W @300 Ω on the balanced outputs that will no doubt be the way to go with higher impedance headphones. Still, this is plenty of power for pretty much anything you could have if in the market for the FiiO K9 Pro ESS, and I even got the notoriously demanding HIFIMAN Susvara to loud levels off just the medium gain setting and XLR output.


I spent weeks using the FiiO K9 Pro ESS with a variety of headphones and IEMs, although I did test the line out features briefly and can state that it works as expected. I felt this source is best paired with headphones, although low gain and a careful hand on the volume knob can get you going with most IEMs, too. The completely dark background helps, easily confirming whether any hissing is coming from the headphones or IEMs. I did not notice any coloring of the sound signature, and measured it to be sure. To do so, I used a few different headphones and IEMs and measured their frequency response from a known transparent source to then compare those when driven off the K9 Pro ESS. Notice the scale bar on the Y-axis—you want a flat 0 dB line from 20 Hz to 20 kHz to match the expectations set forth by FiiO, which is basically what we have here. This also matches FiiO's own published results off various outputs very well, so it comes down to whether you will notice the distortion-related ESS hump as I did occasionally. Still, the ES9038PRO is synonymous with clean, non-colored sound, which attracts many; others may find it soul-less and sterile. I have not heard the K9 Pro with the AK4499; however, it is apparently "more musical"—take that for what you will. Otherwise, I can't really tell you whether the FiiO K9 Pro ESS opens up the soundstage or synergizes tremendously with some headphones given it's a linear amplifier paired with a transparent DAC in all fundamental tones. It doesn't hold back anything I have on hand at least, but I admittedly did have a better time with some R-2R DACs and hybrid amplifiers on higher-end headphones for detail retrieval and some warmth on open-back sets.