Fit and Comfort

Seen above is the right channel of the PMG Audio Apx 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 generally don't find foam tips agreeable for their sound signature. I'd say the Apx is about average in physical size, which in turn means it is one of the smaller tribrid driver IEMs I've tested to date, let alone flagship IEMs which tend to boast large driver counts and associated larger shells too. The inside of the shells is relatively compact without any ridges and a more angled and longer nozzle allowing for a deeper fit in the ear canals. The nozzle is also plenty manageable for most people I dare say, so there is a higher likelihood of the Apx being a comfortable fit for you. The shells weigh ~5.8 g each to make for a low density and minimize any potential of being physically fatiguing. The pre-formed ear hooks combined with the relatively supple cable also makes for an additional point of contact and support for the IEMs, although once again you can see where having flush-mount connectors or even an angled housing would have been better. The vent in the shells help prevent pressure building up in the ear canals to where I had no issues in terms of physical comfort for longer periods of time while still maintaining a good fit and seal. For what it's worth, I did try out some other tips eventually, and I found the Softears Ultra Clears to work very well for my ears and these IEMs too.

Audio Performance

Audio Hardware

There is a lot going on inside the PMG Audio Apx shells, as expected from a flagship set of IEMs. It has a total of 11 drivers per side in a tribrid configuration, although one could argue there are actually four different types of drivers inside if you distinguish between a traditional round planar magnetic driver and one of the newer square planar magnetic drivers. The latter is used exclusively as a subwoofer from 80 Hz down with the rest of the bass handled by a 10 mm dynamic driver—perhaps the same one used in the Custom Art Hybrid 3 Pro? Then come a total of 8 balanced armature drivers for the mids and highs in groups of two each followed by the round planar magnetic tweeter from 10 kHz up for an extended frequency response on either side. The net result is a 1+1+2+2+2+2+1 7-way passive crossover, with each set of drivers positioned carefully inside the 3D printed acoustic chamber and having waveguides for optimal performance. Additionally, there are acoustic filters inside along with the previously mentioned Geometrical Sound Outlet tech to also contribute to the overall tuning. Last, and arguably the most important piece of technology used here, is Custom Art's own FIBAE (flat impedance balanced armature earphone) providing a flat impedance curve to the Apx, which ensures its frequency response will not change no matter the source used. Despite all this going inside, the PMG Audio Apx is one of the easiest IEMs to drive with a low rated impedance of 5 Ω and average sensitivity of 107 dB/mW (@1 kHz). As such, any decent portable DAC/amp is plenty for the Apx and I really don't see the point of going overboard with more powerful gear. That said, more expensive sources can offer different sound signatures which may synergize with the Apx, so your mileage may vary on this front.

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 Cayin RU7 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 PMG Audio Apx. 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 practically identical across the entire useful 20 Hz to 20 kHz range! Keep in mind that resonance peak matching at~ 8 kHz can introduce some measurement artifacts and yet there was no measured difference even worth talking about, let alone perceived in my ears. This is what I expect to see from flagship IEMs that should be using hand matched drivers and undergo much more rigorous quality control, so it is good to see PMG Audio meet these high standards. 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 here too I need to point out such a good match is atypical since the insertion depth can change how the resonance peak and frequency response thereafter behaves even with the fact that these couplers aren't the most accurate in this range. Needless to say, the Apx passes this section with flying colors.



Here is the average frequency response for both channels of the PMG Audio Apx 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 how the PMG Apx is tuned. Note also that I've normalized the plots to 500 Hz in both cases here to better reflect how I hear the PMG Audio Apx—U-shaped, if not warm-bright, with oodles of extension on either side and some excess energy in the upper mids/lower treble.

PMG Audio claims the Apx aims to be natural sounding, but this is one place where I am not sure that I agree. It is a deliberate colored set which makes for one of the most enticing sound signatures and truly worth of being a flagship set that can meet the end goals of many audiophiles. At the same time, as with most such top-of-the-line (TOTL) IEMs, the Apx is not designed to please everyone as much as doing some things extremely well. The customer base who can afford IEMs such as the Apx will no doubt already know what their music preferences are and have a playlist filled with songs and recordings corresponding to specific tracks. As such, I can say the Apx is likely to please many who prioritize great detail retrieval, excellent bass reproduction, and have a good chunk of instrumental music they listen to—especially jazz and classical. Note that the coupler resonance being matched to 8 kHz, or close enough in this case given it is less than trivial to do so, has exaggerated the 4-6 kHz region slightly. But there is no denying that the Apx goes for a bright tuning which may be too much depending on your preference and sensitivity to this region. It can cause some female vocals to seemingly "echo" in the ears and some may even be perceived as being louder than they really are. Those who prioritize Asian pop music and opera singers may want to be especially aware of this, albeit some may well prefer this extra energy for Kpop.

At the same time, this has the potential to have some instrument classes shine tremendously. This worked great for my music library with lots of orchestral music, especially string quartets and piano keys, but not as much for brass instruments and cymbals. The highs being handled by a planar magnetic tweeter also means the timbre associated with triangles and harps may come off slightly metallic and there is also the non-zero chance of the Apx simply having too much energy in the treble too. This is a subjective thing though, I had no issues with it since I use planar magnetic headphones and earphones all the time, but I can definitely see it being fatiguing in the ears over time. Sibilant tracks are also likely to be emphasized here as a result although the Apx itself is not a sibilant set. I tried to EQ the Apx to be smoother in the ears, but it was easier said than done given I kept losing some bass impact (harmonics) as well as the larger-than-life presentation the default tuning results in. So there are compromises either way and I get why PMG Audio decided to keep the Apx tuned as much—those who love it will really love it for the rest of their lives.

I will also give major props to PMG Audio for how the Apx sounds everywhere else. In fact, I can't believe no one has decided to pair a dynamic driver with a planar magnetic sub-woofer to render one of the most satisfying bass experiences I have ever had, even including headphones. The planar driver adds oodles of sub-bass extension with that fast impact I adore, this makes the Apx so good for fast electronic tracks and beats. It leads naturally to the mid-bass where you ideally want some more decay for bass guitars and drum strikes and here the dynamic driver, combined with the ear gain peaking later than you see in my target, helps provide just that. Everything in these regions sounds natural and engaging at the same time. Classic rock, hip hop, country, metal.. I could go on. Male vocals, be it baritones or rap, are also a strong point of the Apx. I also didn't perceive the mids to be recessed either, perhaps this is a result of the crossover involved where the dynamic and punchy bass leads into some of the lower mids too. The end result is a highly satisfying set for a variety of music genres, but less so for others. As long as you know what you are getting into, the PMG Audio Apx is well worthy of your interest.


Direct comparisons to the PMG Audio Apx are going to be other flagship/TOTL IEMs. I have heard plenty of them over the past few months—the Empire Ears Raven, the Noble Audio Ronin, the Campfire Audio Trifecta, the Unique Melody Multiverse Mentor/Mason Fusang/Amber Pearl, the Canpur CP622B, and many others, although these have been primarily at shows and at retail demo locations to where it's not easy to talk about them given the different listening environment and other variables involved. Given this is also the first such flagship set of IEMs I have reviewed, it was not easy to get more in hand at home either. So I decided to start by seeing how much of the Custom Art DNA was in the PMG Audio Apx. The current Custom Art flagship is the 7-BA FIBAE 7 Unlimited, which we've seen before in CIEM form. While not a direct comparison given the Apx is a UIEM set, I do see a lot that the Apx takes and builds upon. FIBAE is included here, for example, and the learnings from subsequent Custom Art sets also make a showing. The snappy bass from the FIBAE 7U is improved upon and made more impactful, with the increased bass shelf being more than welcome here too. The Apx is cleaner in the mids and has significantly better imaging in a wider soundstage too, let alone offering resolution that allows me to pick out details in tracks I only heard before with the Audeze LCD-5.

The Noble Audio Viking Ragnar is a $4000 tribrid set which is arguably going to be even more divisive owing to the tonality. Calling it colored would be an understatement given the Viking Ragnar sounds like a Harman-tuned set which decided to give up mid-way. It also has very good bass and resolution but is a set that really needs EQ to bring down the treble mountain which pierces my ears over time. What a shame given the build quality and included cable are all so nice, and there is so much potential here. I dare say the Apx effectively does all the good things the Viking Ragnar does, but does it better to where I would personally go with the PMG Audio offering every single day. Then there's the Subtonic Audio STORM which shares a similar story to the Apx as being the first release from a new brand of enthusiasts targeting high-end audio. Thanks to Elise Audio for providing a demo loaner for a few days, allowing me to compare the Apx to the STORM directly. The STORM's launch edition cost less than the Apx but is already sold out, with the "regular" edition due to launch soon for a very similar price as the Apx. It aims to replicate the sound of a set of Genelec speakers in your ears and uses a novel pairing of BA and EST drivers, again customized with complex engineering and innovation, to put out a highly dynamic bass, extremely detailed mids, and highs that are smoother than the Apx but not smooth in an absolute sense. Some will find the STORM neutral to their preferences while others won't like it, especially as it tries to do a lot of things. I will say the STORM is more of an all-rounder and a safer buy than the Apx, and many will be interested in the titanium shells too, but it's extremely hard for me to choose between the two for my library. If PMG Audio comes up with an updated take on the Apx which curbs down the upper mids/lower treble without sacrificing on the technical performance, this would be a far easier decision.