What an interesting thread. I came looking for more information on the Snell A iii design and then sort of got hooked on people's reactions.
As an engineer AND music lover I'm well aware that what I am measuring rarely translates into what I WANT to hear when I am fine tuning a product for musical enjoyment. As a result I have to spend a lot of time listening whilst using measurements only to help me uncover something that is audibly irritating. Sometimes I find that the corrections I make in the listening room actually result in a product that measures better. Anyway, I find the whole process of trying to correlate measurement with the enjoyment of music fascinating, which is why I've been doing it for more than 40 years with varying degrees of success (and a few outstanding winners).
There are some conclusions I have come to which others may find will help clear up the misconceptions and disagreements in this thread.
For example a 'perfect' speaker is not defined by a flat frequency response. It may be desirable to have a flat power response but a lot depends on whether it has a smooth Directivity Index. And, unfortunately, none of those measurements indicate how the speaker is going to perform dynamically. But let's take an electrostatic speaker, which we hope (within certain limits) is going to behave well dynamically simply because of the low inertia of its diaphragm. It is actually very difficult to measure the whole output of the speaker because of its size. Which is why some measurements don't look so good (see
https://www.stereophile.com/content/quad-esl-2912-loudspeaker-measurements). Then it acts as a dipole which makes it difficult to measure its power response in living rooms because so much depends on position and furnishings. Even so I have found that most people who come to my demonstrations of the QUAD ESLs find that they can create a vivid impression of real singers and performers in a sonic space which seems to have little to do with the speakers.
Are they then 'perfect'? Not for a lot of people. They are too big for a many people's listening rooms and some listeners prefer the richer sound that you get from drive units in boxes that don't need so much space around them. Which is where the subjective evaluation of what is right for the listener, when he/she/they is listening at home, is so important.
I'm not suggesting that we, as engineers and product creators, shouldn't aim for something that measures well. Of course we should. But we do have to be aware that the standard measurement systems we use bear very little relation to the enjoyment of music. I don't mean, by this, that we prefer, for example, tube amplifiers because they have excessive 2nd harmonic distortion when compared to their solid state counterparts. But it is interesting to note that the best amplifiers I have heard have a smoothly diminishing gradation of distortion from the 2nd harmonic onwards and that's really not surprising when you start looking at the psychoacoustic effects of masking. In any case harmonic distortion is actually quite difficult to hear if it is below 1% - it is the non-harmonic distortions that we should be investigating and who measures those?
Music consists largely of dynamic contrasts - Attack Decay Sustain and Release - with notes that contain a wide and varying arrangement of fundamental and overtone frequencies plus silences between those notes, all arranged so that the timing of all these factors are the elements that give us music's emotional impact and enjoyment. I contend that not only do we currently have no way of measuring this but that our measurements are sadly lacking in analysing how any piece of equipment relays the complex sonic patterns that we call music.
So, choosing any piece of hi-fi equipment based solely on whether it measures well does, I fear, often lead to long term listening dissatisfaction. Whereas, in my experience, choosing equipment based on finding out which gives the most musical satisfaction usually leads to long term ownership without the seemingly continuous urge to 'upgrade' which affects so many 'audiophiles'. Of course, if your preference is to choose equipment which both sounds good and measures well, go to it. But don't expect good measurements on their own to necessarily result in a system that gives you long term musical satisfaction.