Have you also heard the Technics btw?
I've had extensive experience with the LS50. The discontinuity is highly audible on the LS50, I called a peak at 2kHz - in multiple listening rooms - even before any measurements came out. The elevated listening window response and off-axis discontinuity contributes to a subjective sense of an obscuring sheen on brass, accentuates sibilance and emphasises hiss. Nonetheless I do not regard any sighted listening impressions as binding or relevant to evaluating the performance of a loudspeaker.
I have not heard the Technics because Panasonic doesn't intend to distribute it in my territory, but if you are looking for people who have, there
were a couple on DIYAudio whom I turned onto these speakers, who have also heard the LS50 and compared it.
I've heard neither, and personally from the measurements I would consider both speakers to be quite well-performing although not perfect when it comes to polar response. Sure the, discontinuity in polar response is more pronounced in the KEF, but it's also much narrower in bandwidth. Hard to make a definitive call from my POV.
In any case, my much greater concern with both speakers would be that they are small passive ported speakers with port tuning frequencies up near 50Hz, and what this would mean in terms of diaphragm displacement and port compression (and resulting distortion) when fed signals with significant content below 50Hz.
On the bandwidth of polar discontinuity, the KEF spans 800Hz to 5kHz (over 2 octaves), if we include the weird early beaming and widening of the midwoofer below the crossover. The LS50 has a dramatic dip beginning at 800Hz centred at 1.3kHz that deepens off-axis, and causing 2kHz to be relatively peaked when dispersion broadens again.
The Technics doesn't suffer from the 1.3kHz beaming and broadening, it has substantially smaller variations spread from 3-9kHz (about the same octaval bandwidth). I'd personally take smaller variations higher up in the range than severe discontinuities in the upper midrange and low treble.
Another interesting thing is the unusual port designs both have. The LS50 has semi-permeable port walls, the Technics apparently has a port that faces a
"padded partition" internally and so there is no port resonance in the midrange that plagues some speakers (including the JBL LSR705 at 800Hz).
edit: yes, porting might be an issue, but it isn't inherent to coaxial speakers, but affects all speakers of that size class and tuning. Though you have a point insofar as excursion of the midwoofer modulating tweeter output.
edit2: might I suggest that
@amirm or someone else spin off this coaxial discussion into a separate thread? It is a derailment of the original thread but I think the discussion on the properties of coaxial drivers is pretty good.