There's no doubt — Thieaudio's Monarch MKIII is the best IEM I've used yet

Thieaudio needs no introduction; Linsoul's in-house brand debuted on the six a half-decade ago, and it released several hits in that time. The Oracle MKII showcased what it can deliver in the mid-range, and it built on that with the Prestige LTD, a high-end offering with a unique tuning.

But it's the Monarch series that defines the brand; the Monarch MKIII was introduced at the end of 2023, and it retails for $999 on Linsoul Audio. Like the Fiio FX15, the Monarch MKIII uses a tribrid configuration of drivers that includes dual dynamic drivers, six Knowles balanced armature drivers, and two Sonion EST drivers within each shell. Combine that with Thieaudio's sound signature, and the Monarch MKIII continue to be a phenomenal choice in 2025.

It's clear that Thieaudio wanted to differentiate just about every facet of the Monarch MKIII, starting with the design. The IEMs are sold in a standard resin shell, for an additional $100, you can heavily customize the design. There are six shell colors available, and 14 different faceplate options, giving you extensive customizability that you just don't see with other IEMs in this category.

The only downside to going with a customized design is that it takes anywhere between two weeks to a month to get the product, but that isn't too bad considering you get a unique style. I got the standard variant, and while it doesn't stand out quite as much as the custom colors, it still looks quite good.

The shell is made out of a resin material, and you get the usual stainless steel sound tube. Build quality is excellent, and I didn't see any issues in the month I used the Monarch MKIII. Fit is quite good too, but the IEM is on the bigger side as these things go, so you may see issues with the fit if you're got smaller ears. That said, it has a contoured design that ensures it stays in place once you wear it.

You get a selection of foam and silicone ear tips with the Monarch MKIII, and it isn't any different to what's included with other Thieaudio IEMs, which is a shame. Fiio does a much better job in this regard, with its high-end FX15 coming with double the number of accessories.

You should still get a decent fit from the options included in the package, and Thieaudio's bundled cable is of a good quality. It connects to the buds via the standard 2-pin connector, and there's an interchangeable plug at the other end that lets you switch between 2.5mm, 3.5mm, and 4.4mm with relative ease.

I like the case Thieaudio provides with its products; it has a durable design and does a good job safeguarding the IEMs and the cable. The packaging itself is suitably high-end, and Thieaudio did a good job in this regard.

The standout with the Monarch MKIII is the sound; Thieaudio deserves credit for creating a dynamic sound that shines across various genres. Having used the Prestige LTD for the better part of a year, I was immediately excited with the low-end extension on the Monarch MKIII; it does a much better job in this area, and you get a definite rumble.

There's good vibrancy and energy with both sub-bass and mid-bass frequencies, and it allows greater definition to come through. The mid-range has clean articulation, and vocals shine in particular. Instrument separation is good as well, and ensemble music — like orchestral pieces — sounds great on the IEMs.

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Treble has good extension, and at no point did I notice any sibilance. Thieaudio did a great job with the dynamics and soundstage, and the Monarch MKIII has a sense of airiness and warmth that makes listening to music that much more enjoyable. Unlike most IEMs in this category, it is particularly well-suited to metal and thrashcore.

It doesn't take much to drive the IEMs either, and I used it with the Fiio K19 and the M15S audio player. On the whole, the Monarch MKIII thoroughly deserves its status in this category, and I didn't think I would enjoy using the IEMs as much as I did. If you need high-end IEMs with a customizable design and groundbreaking sound, the Monarch MKIII are a no-brainer recommendation.