I think WTS gives a very reasonable description of the differences between the 57 and the 63 speakers.
For a short while I had both types in my house at the same time. I could easily enjoy either speaker, but for me the 57 wins on the points in replay most to my taste. For me it is crucial that the speaker must work quietly as well as louder. I never want “really” loud!
Eventually I went to using a single ESL 57 in a pure mono arrangement. Of course there are some problems that arise, but on mono recordings this is probably about as fine a speaker as can be had, unless you want a more PA style. I actually like the fact that the bass side never threatens to swamp the middle and soprano lines. Many recordings contain a bass weight that is out of proportion so the speaker manages to bring this back toward the natural without any damage at all. However the speaker never sound thin or weedy even on quite light toned recordings. I am not sure how that trick is managed.
There is a good way to test if a speaker has a proper address of the bass. Find a piece of Bach organ music, played on a baroque period organ, with the use of the lowest notes on the pedal board. The official lowest note is the sixteen foot “C” which is tuned [depending on the overall pitch of the organ tuning] in the region of 32 or 33 Hertz. If these lowest musical notes are well defined and in balance, then the speaker is doing its job. These old organs do not thunder in the way of much larger late nineteenth century organs can so easily do, but they are notable for clarity and articulation, and firm focus on the pitch.
I have heard even very expensive coned driver speaker make meals of some of these notes, while the next note a major or minor third away can be quite subdued. One thing the ESL has that is gratifying is that though it undoubtedly does roll off at the frequency extremes, it does so very very evenly.
Probably the least flawed speaker yet made, though with enough limitations to make it impossible to call a universal recommendation. I suspect that once the ESL qualities get under the skin, it does become impossible to find anything else quite as lovely!
ATB from George