Suffice to say that this most likely not caused by your mains. Buy different monitors w/o class D built-in amp. Or better, buy un-amped "monitors" and build/buy a linear amp with a linear power supply. Unless you plan to demolish the walls of your flat (with SPL), chances are you won't notice the difference on your electricity bill. And I'm guessing you didn't make spectrogram yourself, or you wouldn't be asking this kind of question.

As for balanced connection... if it makes the noise without any signal source/cable plugged into it, then that surely won't fix it. Otherwise, how good is your source in terms of SNR? Can you listen to it (your source) with anything else?

@RespawnedFluff First off, I'm not sure what ‘kind of a question’ do you mean but I did make the spectrogram myself (not sure how it's relevant, though). Second, you provide no useful explanation that would help me understand the issue. The SNR of my source is 115 dB and yes, I can listen to it with something else… without the noise. Finally, I'm obviously trying to understand the origin of the noise and why some people hear it and others don't, not merely solve my issue by buying something else. Thanks.

The origin of the noise is the amplifier itself then. A class D amplifier is switching circuit, quite capable of generating high-frequency noise precisely of the kind you've posted in that spectrogram.

Even when powered from a battery which gives "clean" DC, you an get noise on a class D amp, e.g. youtu.be/STyUxXCIE5o?t=225

@RespawnedFluff So are the people who hear it only when they put their ears right in front of the tweeters practically deaf or is there perhaps some other variable that could render this feature more pronounced? Perhaps ethernet over power (EOP)? Thanks.

There is substantial variation between people in sensitivity to high frequency sounds [and noise]. I'll see if I can a paper on that, but basically the older you are, the worse your high-frequency hearing gets. But it's very late where I am and tomorrow is a Monday.

@RespawnedFluff Do you have a moment, please? I've bought another pair of monitors with Class AB amplifiers and they hiss as well… It's the same kind of hiss, maybe not as strong. Is this normal too or could it be related to improper grounding (I'm asking because my USB ports and onboard audio outputs conduct noise affected by CPU and GPU utilisation, mouse movement, keyboard backlight, etc. even through an external DAC and it's been suggested to me that it may be a grounding-related issue)?

Could be normal (or not) depending on their SNR spec. Please post another question with those, with details. If I got this right, the noise varies with PC activity? If that's so, you do have a problem with (PC power supply switching) noise being transmitted to the amp/monitors. You need to have at least one good common ground to avoid that, but it may not be enough, depending on how much noise is being generated and the noise rejection spec of the amp.

For example, if I put a 10K/10K resistive voltage divisor out of an E-MU 1212m, and I pick the signal 10K lifted from the ground (i.e. in parallel to the high-side resistor), I get noise in my amp, even though this card has an amazing 120db SNR. But if I pick the signal with a common ground (i.e. in parallel with the low-side resistor): no noise whatsoever. And this even with a lowly TDA2030A amp made on a stripboard (which needs a resistive divisor because it has high input sensitivity, in case you wonder how I came up doing that).

Also, having ferrite chokes at both ends of an unbalanced audio cable helps somewhat when you take the signal out of a noisy PC source (I'm not talking about the E-MU here).

@RespawnedFluff Okay, thanks. Another question is coming up. Some of the noise does indeed vary with PC activity (more like a buzz with changing frequency and amplitude), some doesn't (the ‘Class-D hiss’ in the JBLs, the Pioneer amplifier from the video that you referred to, and now the Class-AB KRKs). I've already tried a single ferrite bead and the effect is minimal, sadly.

If the monitor has any sort of signal source or even a cable connected, then you don't know for sure where the noise is coming from. If it's connected to a computer then the source could very well (in whole or part) be the computer. Try your experiment again with pins 2 and 3 of the input XLR shorted together.

It's worth pointing out that Class-D amplifiers do not inherently have some sort of unavoidable hiss. While all amplifiers will have some sort of noise (see Johnson noise), well designed Class D amplifiers can have very low noise floors. There are many products with class D amplifiers that have almost imperceptible hiss at full gain. It's not so much that you need "non class D" amps, you just need better-designed monitors.

The best example I can give is comparing Mackie SRM450s with QSC K12s. I used to run the Mackies as tops supporting orchestra; during concerts, the hiss of the Mackies was audible at the back of the venue when no one was playing. Switched to K12s and the hiss was only audible when I was ~10ft away. Both are Class-D, the K12s simply have a much lower noise floor.

@uint128_t That's very interesting, thanks for sharing your experience. I wonder why would a company like JBL with a huge R&D budget and presumably lots of experienced engineers design their speakers without this in mind. I'm assuming that material requirements to ensure there's no hiss can't be significant—or are they?