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The sweet spot with 57s (in stereo mode George
) is just too tiny. In an ideal world I’d have kept both, but didn’t have the space.
You absolutely make a crucial point. In stereo the ESL57 has a difficulty in terms of there being a rather small [but ever so nice] sweet spot for stereo imaging. Enough for one person in the right place.
In mono this really ceases to be a problem at all. Of course the 57 really does beam the higher frequencies in a narrow arc straight out, so that off axis the sound does become softer in balance. In fact I find this useful as on an over bright recording all I do is move slightly off axis to correct. Otherwise I sit in the straight forward position.
Another aspect of the original 57 is that it is tilted back and sits low to the floor on its original three little feet. If you sit too low then the beam of higher frequencies may go right over you head [if you are far enough away], so the solutions may include raising the speaker on higher [aftermarket] stands, sitting on a higher listening chair, or tilting the speaker forward [more towards vertical] with something like a small coin, or cut down wine bottle cork [to make a sort of cork washer], and this brings down the beam to ear level.
I prefer to avoid raising the speaker as in doing so you reduce the floor re-inforcement of the lowest bass tones, which can bring a rather light-weight effect.
I used to have a cork prop on the back foot, but now I have a comfortable high chair, which is a good solution.
Well set up with plenty of space behind the 57, and it not radiating perpendicular the wall behind, I still believe the 57 is the best mono speaker ever made. The 63 is not a satisfactory mono speaker, because of the idealised point source system in the panels. This leads to a terrible small mono image, whereas the 57 is not a point source design and radiates a very natural and easily enjoyed "wide" mono presentation.
The very fact of the 63 idealised point source design is that it works very much better for stereo over a bigger sweet spot, so that two people can sit beside each other and still get a good stereo effect, which is nigh impossible with a pair of 57s.
Hope that these observations help.
Best wishes from George