Since I got mine in the beginning of the week here is my compact review and thoughts as well as an (unfair) comparison.
Disclaimer: Below text reports my individual perception in my listening room, with my own habituation and preferences, yours can differ significantly.
For the last 3 days I was spending every day few hours listening to them, without doing any measurement or reading any other reviews (I had read the 2 important ones when they were released but didn't remember many details except that Amir quite liked them and Erin found them a bit too mellow) to hopefully not have too much bias at my listening.
First thing I noticed is that like most compact loudspeakers they don't have much deep bass (especially in my quite bass absorbing room) so I moved them quite close to the front walls, placed in 2 meter stereo triangle (on 60 cm high stands which bring my ears in a height between their woofer and tweeter), towed horizontally around 10° out compared to my ears. Placed there with some tracks they positively surprised me with their bass, especially considering their small woofers (which by the way around 1 cm smaller than the LS50 ones, although both nominally sold as 5 inch).
Listening to couple of loud sine sweeps also I didn't hear any pronounced resonances which is something not automatically granted in this price class, also the finish is impeccable. Imaging was as with most well engineered compact loudspeakers good.
Tonally I didn't find them mellow or having a typical British tuning like for example other Wharfedale models like the Lintons (which are tuned by Peter Comeau, while the Diamond 12 series was tuned by Karl Heinz Fink), but rather the opposite, especially somewhere in the upper mids / lower treble there was something that made them a bit too aggressive/shouty for my taste. To reduce the chance of just having a bad mood or acoustic fatigue I nowadays repeat the listening for at least 3 days which I did, in this case my impressions didn't really change. The upper treble which was also not shy didn't disturb me, in contrary it gave together with the impressive for the size bass a quite joyful character.
To make sure it is not just my personal habituation after these 3 days I directly swapped them (keeping the positions identical) with a pair of KEF LS50 Meta. Well, that aggressiveness was gone, in some tracks I would prefer though the slightly accentuated upper treble of the 12.1.
To find the causes I made some quick listening position measurements of both as there already exist enough great quality anechoic measurements of both. Some might rightly say that from price point of view that comparison is unfair and I will agree, I used the LS50 Meta as I know its sound well and I rather see it as an anchor point for comparison in that size class.
First I measured the multitone distortions (as I find them better correlating to audibility) at a quite high listening level on my listening position with both L and R playing, giving a total SPL of around 85 dB and spectral peaks of over 100 dB:
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It is interesting (and impressive) that till approximately 300 Hz the 12.1 seems cleaner while above that the LS50 is superior. The higher distortion there could have been one reason for the aggressiveness but as it happened also at lower listening levels I think its influence is not dominating. (Must say both play quite good loudly at a 2 meter stereo triangle, for larger distances I would recommend subwoofers and highpass filtering them.)
Now lets have a look at the frequency responses in a 40x40x40 cm cube spatial average around my listening position with the moving microphone method, I use below 1/6 octave smoothing for easier visibility:
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It can be seen that the 12.1 has some extra mid bass which I enjoyed in my rather bass absorbing room and its from other reviews infamous dip between approximately 1.3 and 2.3 kHz which can give it according to some a mellow character, something I also expected when seeing the measurements after. But it also shows a peak between 3 and 7 kHz which was the reason of my annoyance. When I took it down a couple of dB per PEQ it was fine, although still a little treble heavy for high SPLs, personally I would prefer the upper treble for many songs somewhere between both loudspeakers.
That's for me also a good lesson that the predicted in room response is not always sufficient to see such issues
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but it is good to check also for other measurements like the sound power, especially at increasing listening distances where it is correlating more to the listening position measurements and impressions:
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(source of the two last plots
https://www.spinorama.org/ )
As a conclusion of this quick and short review I can wrap up that the 12.1 impressed me, even more considering its price here (I got a new pair for under 200€) but on the other hand it is a loudspeaker which in my current listening situation I wouldn't enjoy listening without equalisation even above the modal region (same for the old KEF LS50 non-Meta so this is nothing to be ashamed of), so despite all the love I have for the objective side of audio I have to speak out also a warning of buying loudspeakers by just viewing few measurements and scores or listening to even experienced reviewers (which have possibly a quite different listening situation), especially if someone doesn't want to or cannot use equalisation.