Fit and Comfort

Seen above is the right channel of the ThieAudio Monarch MKIII placed into an artificial ear mold with the included size M silicone ear tip installed. I have average-sized ears, and the ear mold above represents my own experiences well enough as a proxy. Size M silicone tips are my go-to for testing, since foam tips are not included by some brands, and I found these tips to work fine enough even if they were slightly stiffer than I personally prefer. I eventually ended up going with more malleable tips such as the Softears UC. The Monarch MKIII is a larger-than-average sized set that will jut out of the ears for pretty much everyone. There is also a decent chance those with smaller ears and/or ear canals won't find the larger shells and nozzle comfortable. The consolation is it's not that much larger than most IEMs on the market today and on par with those that have as many drivers inside. The ergonomically designed shells also help as does the nozzle insertion depth and angle itself. I suspect a bigger issue is that protrusion at the top which may or may not work for your specific ear shape—it was plenty fine for me and even aided in a snug fit. The shells being made of resin also means there isn't any physical fatigue from weight either. The end result for me was a set with multiple points of contact in the concha as well as the cable providing further support on the back.

Audio Performance

Audio Hardware

The ThieAudio Monarch MKIII employs a tribrid driver configuration in that there are three different driver types here for a total of 10 drivers per side. The predecessors both had a 1 DD/6 BA/2 EST driver configuration and this bumps things up with a second dynamic driver while keeping the rest the same, if not exactly the same drivers too. ThieAudio provides very little details here aside from calling the dual dynamic driver setup as "Impact²" and these are supposedly in an isobaric configuration as with the likes of the 64 Audio Volür we saw recently. We do know these use 10 mm composite diaphragms and handle the lower frequencies with the balanced armature drivers—a Sonion 28UAP and two Knowles DWFK 31785 dual driver units—cater to the mids and mid-highs. As expected, the Sonion electrostatic tweeters help with the higher frequencies to make for a 1+2+4+2 4-way crossover. The Monarch MKIII has an average rated impedance of 18 Ω and slightly below average sensitivity of 99 dB/mW to where it is easy enough to drive off any decent portable DAC/amp on the market—there is no need to go with the balanced connectors either unless you just like how the 4.4 mm connector looks and feels.

Frequency Measurement and Listening

I will mention that I have a general preference for a warm-neutral signature with a slightly elevated bass, smooth treble range, detailed mids, and good tonal separation. I also generally prefer instrumental music over vocals, with favored genres including jazz and classical music.


Our reproducible testing methodology begins with a calibrated IEC711 audio coupler/artificial ear that IEMs can feed into enough for decent isolation. The audio coupler feeds into a USB sound card, which in turn goes to a laptop that has ARTA and REW running and the earphones connected to the laptop through a capable and transparent DAC/amp—I used the Questyle M15 here. I begin with an impulse measurement to test for signal fidelity, calibrate the sound card and channel output, account for floor noise, and finally test the frequency response of each channel separately. Octave smoothing is at the 1/12th setting, which nets a good balance of detail and noise not being identified as useful data. Also, the default tuning was used for testing, and no app-based settings were chosen unless specifically mentioned. Each sample of interest is tested thrice with separate mounts to account for any fit issues, and an average is taken of the three individual measurements for statistical accuracy. For IEMs, I am also using the appropriate ear mold fitted to the audio coupler for a separate test to compare how the IEMs fare when installed in a pinna geometry instead of just the audio coupler. The raw data is then exported from REW and plotted in OriginPro for easier comparison.


The IEC711 is such that you can't really compare these results with most other test setups, especially those using a head and torso simulator (HATS). The raw dB numbers are also quite contingent on the set volume, gain levels, and sensitivity of the system. What is more useful information is how the left and right channels work across the rated frequency response in the ThieAudio Monarch MKIII. The left channel was separately tested from the right one, and colored differently for contrast. I did my best to ensure an identical fit for both inside the IEC711 orifice, so note how the two channels are identical across the entire useful 20 Hz to 20 kHz range! This is good enough to where it does merit being called a flagship set if this is the level of stringent quality control and channel matching expected out of all Monarch MKIII units. Given that the custom design versions are made to order, they may even get more hands on attention throughout the manufacturing process. Measurements taken after 100 hours of testing, which included these playing a mix of various songs as well as white or pink noise and sine sweeps, showed no difference. There was no perceived burn-in effect thus, and none was measurable, either. The response with the anthropomorphic pinna in place matched the ideal scenario in the coupler extremely well too and this is an indicator of how good the fit and seal was when installed in the artificial ear. But keep in mind that the pinna doesn't care about any comfort issues, so your mileage may vary in your actual ears.



Here is the average frequency response for both channels of the ThieAudio Monarch MKIII plotted against my personal target taken from VSG.squig.link, which also gives you an idea of my personal preferences to better correlate any possible biases. The tuning of a set of headphones or earphones does not have to match my target as long as it is tuned with some direction, makes sense, and is executed well. After all, no one set will appeal to everyone, and having different options is what makes this hobby so interesting and hard to quantify. This is in addition to a second graph using a newer target that's based on a more scientific methodology involving a -10 dB tilt (-1 dB/octave) applied to the diffuse field target for the newer, more reliable B&K 5128 but then compensated for my exact 711 coupler instead. Do scroll down to the targets in my database linked above and see the new 5128 section to the left where you can click on the yellow question mark for a brief primer. The bottom line is this target is closer to what many people are likely to prefer out of IEMs and headphones alike. I have chosen to plot this graph with the frequency response normalized/compensated to the target to make it easier to see where the Monarch MKIII deviates from this target. This helps visualize the various elements of the set's tonality—elevated bass, mostly flat and steady mids, relatively smooth and extended treble.

You know what? I do think ThieAudio has addressed the bass quantity and quality issues the original Monarch and Monarch MKII had. The newest Monarch MKIII has a gentle bass glide starting from 250 Hz itself which becomes a full-on bass shelf of ~9 dB with excellent extension all the way down to 20 Hz and below. This means you have decent mid-bass too, thus reducing the potential for people to find this to be a lean-sounding set. In addition, the bass also hits quite hard while also being nicely textured and detailed—this may well be down to that dual DD setup. A good fit and seal pretty much guarantees an impactful (get it?) bass presentation with enough punch and slam for people. The bass also comes across like a better implementation of the Harman 2019v2 in-ear target in that you have a touch more mid-bass and no pronounced bass tuck while still keeping it fairly clean and not affecting the mids clarity. Speaking of which, I am glad ThieAudio's tuning team continues to do a stellar job here. The original Monarch won my heart by offering a midrange unlike any other set with so much capacity to handle vocals and instruments.

We get that same tuning philosophy here with an extremely flat response up to ~800 Hz allowing great instrument separation and accurate music reproduction even in extremely busy tracks. Imaging is precise, I have no complaints on the detail retrieval front for this price range, and soundstage feels correct enough even if personally I would have liked something wider and deeper. This does feel more like a set of monitors than a deliberately colored midrange for fun. It continues with the ear gain region too where we see a straightforward hike from 1-3 kHz of ~7 dB—this does feel like ThieAudio decided to play it safe and go with the agreeable tuning of its previous IEMs rather than look at what the more accurate B&K 5128 systems would indicate though. The recent trend of sustained upper mids to try and lower any ungainly peaks causing fatigue and sibilance continues here. Personally I thought it was a bit too much upper mids energy which also affects timbre slightly. On the flip side, this does make vocals shine with excellent harmonics even for those among us preferring Kpop while also adding some extra ooomph to string instruments for acoustic jazz lovers. The treble response does feel weaker in execution compared to the rest though with ThieAudio still not having properly conquered the EST drivers for me. Maybe it is time for them, and many other brands in fact, to consider going with open-cavity BA drivers or micro-planar drivers for the highs instead? I found the added air to not hit the mark with the likes of harps and cymbals still feeling flat. It also clearly didn't do much for increasing the perceived soundstage either. There is a bigger chance of annoying people with the upper treble presence—something ThieAudio also did with the more expensive Prestige before it went with a tamer revision in the form of the Prestige Ltd—rather than the Monarch MKIII making my recommended list for classical/orchestral music.


The obvious comparisons for the ThieAudio Monarch MKIII would be against the previous two ThieAudio Monarch sets. Keep in mind that the original Monarch sold for less and is discontinued now. You can still purchase the Monarch MKII as of the date I write this review and it costs the exact same as the non-custom design Monarch MKIII. Perhaps you might do this if you like how that set looks. There is also something to be said about the MKII having more of an all-rounder tonality. It certainly is easier on the ears in the treble while being pretty much the same when it comes to resolution in the highs. But the Monarch MKIII is a good step up in the bass and mids for reasons previously mentioned. Bass guitars don't feel left out any more and the BA driver implementation in the mids also seems different to where vocals were more forward facing with the newer set. There's also improved bass dynamics and the various custom designs also help. I also think the new cable is an improvement over the previous one although the Monarch MKII might be more comfortable for most people since it is less thick and doesn't have as pronounced a bump on the top. The original Monarch also tried to do something special by being a mid-centric set with a sub-woofer bass response and did it well too. If all three sets still sold today at their asking prices, I'd go Monarch MKIII>Monarch>Monarch MKII myself.


The tougher competition to ThieAudio is from other brands though given there have been some fascinating entries recently to make the ~$1000 market more competitive than ever before. Some are slightly more expensive—here perhaps ThieAudio's Prestige LTD may be worth considering too but I don't have a unit here to really say more about it. Just as the Prestige LTD is a differently tuned version of the Prestige, which I thought necessitates EQ to make the most of it, Symphonium Audio partnered with local UK retailer Elise Audio to put out a limited version of one of my favorite sets, the Symphonium Helios. The newer Helios SE adds a bit more mid-bass and re-works the treble while also reducing the shell size significantly to address multiple issues the original set had. I do feel the Helios SE is less of an all-rounder than the Monarch MKIII though, especially with it still having a bass tuck, but it does a better job in the higher frequencies and offers better accessories. It goes for a metal shell with a blue color to the face plate so the Monarch MKIII bests it in terms of how many designs there are, although I like the cleaner, more durable shells of the Helios SE too. The Monarch MKIII does cost less though and is more readily available so I'd probably say this is the way to go for most people.

The Custom Art FIBAE 5 is an interesting set which launched an year ago with a Knowles damper-based customizable bass shelf. It is best seen as an eccentric set these days though wherein I am not sure the feature set and tonality is going to be a top recommendation anymore. In particular, the upper mids and treble can come off quite hot and peaky to be fatiguing and the micro-planar drivers used for the treble can be hit or miss. It does work quite well for a few music genres and the adjustable bass shelf would have been worth talking more about but then came along another set which does that but better. The toughest competition to the ThieAudio Monarch MKIII, as far as I am concerned, comes from the 64 Audio U4s. It's not as dynamic or engaging in the bass as the Monarch MKIII is—those dual DDs work quite well, the much more expensive 64 Audio Volür is the only set from the US-based brand which does better in this regard—but bests the Monarch MKIII in offering a customizable tonality thanks to its Apex modules. There are four modules which come with the set which change the tonality of the U4s from nearly L-shaped to almost neutral with a balanced tonality as well as a warm-neutral tuning being the other two options. I also liked the treble presentation of the U4s more than that of the Monarch MKIII, and the smaller size can be more comfortable too. ThieAudio wins on the design front unless you are again a fan of the blue metal shell aesthetic and the long U4s nozzle can be a pain for some people whose ear canals have an early bend. Likewise, the Monarch MKIII is more detailed and arguably works better for vocals even if it can be a bit too much in the upper mids. The U4s costs $100 more compared to the stock version of the Monarch MKIII though, so keep that in mind.