You might want to look at the Naim product white papers on the ND555 and Naim DAC if still available.
The SHARC is the programmable digital signal processor made by Analog Devices that Naim use in their current digital source products.
No doubt all engineering including software engineering is often creative… I am an engineer and I can definitely testify to that… 
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Here you go;
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If it’s audible then in principle it should be measurable, if it’s not objectively measurable then the chances that external factors such as @davidng mentions (listening environment, temperature, humidity, air pressure or even having a slight cold etc.) will be more likely causes. We can expect Naim to do the proper measurements and release firmware that has been tested within spec, which means that any remaining audible difference are statistically more likely to be circumstantial to the listener.
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Indeed, even if the measurements are undertaken by humans using scientific Mean Opinion Scoring methods… and that is how Naim tune, along with many manufacturers who fine tune products to suit human consumption. In my professional world we also follow standardised methods for MOS, in parallel with human sampling.
The real interesting challenge is modelling MOS, but that is a whole other topic
The Harman Curve is an interesting example of this in the audio replay world.
Yes, but that is not how Naim do it… or I can definitely confirm it wasn’t how Naim did it a couple of years back, in terms of firmware tuning.
Firmware is audibly checked prior to release, and yes there can be subtle differences between firmware releases despite this.
Sure independent of this, humidity, sound pressure, listener mental state, temperature can all affect how a sound appears to us.
Humans in groups do tend to display psychological group biases towards certain experiences (think shared religious experiences for instance), and the statistical sample size to reliably draw conclusions from subjective reports is likely too low on the Naim forums.
Wat could be interesting is if people submitted their subjective reports on firmware sound quality anonymously and independently from each other, so that peer influence is minimized, and then compare that with a scenario where people give their reports in a public environment such as on the forums. It would be interesting to see if a public environment would increase the number of comparable reports (better/worse sound quality), and to what degree.
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Indeed… turn the heating up, have a glass of wine, and it’s amazing how euphoric your audio can sound
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Well bias, and subjective bias, can invalidate results if done in an amateur manner.
We follow these methods such as ITU P800 amongst others
https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-P.800.1-201607-I!!PDF-E&type=items
The AES has interesting papers on measuring emotional responses to sounds as well, using similar assessment methods.
TY @Simon-in-Suffolk and @Richard.Dane, read both articles. I only learned one thing from them: Respect to the designers / engineers at Naim.
Send through if there are any similar articles for the Uniti range, please. 
Sorry, late to this party. But I think adding tone controls is absolutely ridiculous!
I would rather see Naim add filter options like other DAC makers do, like dCS. I’m not sure what Nam uses. Perhaps minimum phase? Different listeners would have different preferences on which one sounds better to them. On the other hand, I can see this starting endless discussions on which one is best.
I don’t understand… you can undertake tone controls in digital or analogue domain, each has pros and cons… you say it is ridiculous and then go on to advocate it.
I wonder if you are perhaps confusing reconstruction filter types that you see on some DACs with eq tone controls… they are chalk and cheese.
Not confused. Yes I’m talking about reconstruction filters.
Being a DCS user, i can say that the filters are a useful tool, and so is having 3 different dac maps to play with and being able to change the up sampling as well.
Most end up using the same for all, but it takes some time to get used to what each does, and what to listen out for in each one, as they are not big changes like tone controls, but its great to have and you can certainly find the settings that best suits you
Ok, but reconstruction filters are not do with tone controls or eq. Their function is quite separate.
Reconstruction filters act as low pass anti aliasing filters and in higher end DACs can adjust for the sinc function. DACs sometimes offer alternate reconstruction filters so you can choose the one that produces least distortion or artefacts in terms of your preferences around the filter cut off point.
Good point/distinction here @Simon-in-Suffolk. I believe the Rega Saturn-R’s DAC that I use for the ND5 XS2 provides the latter of what you mention—4 of them. Similar to what @Dunc mentions, it does take some time getting used to what each offers. I, personally, went for ‘filter 4’ and haven’t really looked back. It explains in the literature that for medium and high sample rates (88.2/96 & 176.4/192K), filter 4 is a minimum phase apodizing filter. Doing some reading, I take this to mean that it “undoes” ringing of other filters in the A/D/A chain, and is basically referring to the filter with sufficiently gentle frequency slope whose stopband starts lower than the transition band of other ‘ringing’ filters. Like any available options there are always trade-offs. With this one you may lose some perceived frequency extension from my understanding.
In this review of the ND5SX2, https://www.stereophile.com/content/naim-nd5-xs-2-media-player-measurements I find it disturbing that the frequency response is not extended with the higher sampling rates.
I don’t know if this was only done on the entry level player to reduce cost, or if this applies to the NDX2 and ND555 as well. I hope not. It does seem we’re stuck with the one filter choice. 
If talking about Naim, the filter is carefully designed and implemented, both the reconstruction and analogue low pass.
Read the product white papers to see how they were designed to sound optimal. You certainly wouldn’t want to have different filters.
The analogue output filter on the ND555 is achieved using a sixth-order Butterworth filter, realised as two third-order active filters in series, as first used in CD555 and subsequently Naim DAC. The filter is the same on lesser DACs, but more modest electronics are used. The output filter is fixed irrespective of the sample rate used.
Seems to be -3dB at 30 kHz, up to this point very similar to the traditional roll-off of Naim amps. The 300 DR:
Looks less dramatic due to the scales differing and the slope starting at ~10 kHz, but it is also -3dB at 30 kHz. I understand that Naim has done this forever in the amps to avoid spending the amp’s resources in operation on frequencies nobody hears. When the amps do it, it seems useless to force the streamer to do something different. Of course in the streamer response it goes to zero at essentially 30 k, but I suppose this is the result of the filter that sounds the way they want and would have designed it run higher if it sounded better. Of course if the streamer went to 40 or 50, there would not be much left after the amp for some supertweeter to use.