How do you know what a good speaker is?

Frequency response for high end speakers at every price level is usually relatively flat. The differences in audible sound quality reported by audiophiles is disproportionate to the differences in frequency response between different speakers therefore frequency response cannot be a very significant factor in what we're hearing.
Distortion is usually below 0.5% so again the same reasoning applies.
I'm not convinced that polar response is quite as important as is sometimes claimed. 

If you look at the specs of most ultra high end loudspeakers,  there's no hard evidence provided by the manufacturers to justify their claims that their speaker is vastly superior.
And if there was it would need to be independently verified.

So how does the consumer know how close any given loudspeaker is to the ideal loudspeaker? How do we know how close a loudspeaker is to recreating the sound of a violin, cello, piano, human voice, or anything else? 

What makes a magico vastly different from a yg or Wilson? On the other hand if the difference between these speakers is extremely small then why is there such a discrepancy in opinions and why do we need a yg and a magico and Wilson and tidal audio and b&w etc on the market if they're all so similar?