It's hard to describe. I"ve had the ESL63s from new since 1990, originally in a Quad CD66/34/FM4/606 system and felt no need to change the speakers when I upgraded to my Naim source and power system in 1999. They are wonderful speakers. But........
I had heard the ESL57s years ago as a schoolboy, when my father and I went to a music society. Terrible music, but the speakers sounded wonderful.
Once I could afford to buy Quad speakers, the earlier ones were no longer made, so I got the ESL63s. But I carried on remembering........
And now, with eBay, you can pick up a reasonable pair of ESL57s, for £400 plus/minus£50. I took the plunge and loved what I heard but guessed that, with a pair of speakers 20 years old, I couldn't be hearing what they might have sounded like new. At which point, being very silly, I bought a second pair and arranged to send both pairs to Quad Musikwiedergabe in Germany. They tell me that the treble panels of one pair were completely burnt out, that the treble panels of the other pair were down and that all four pairs of bass panels varied and were much too soft, compared to bass panels as new.
So................... I swallowed hard and gave instructions for a complete rebuild.
I have little idea what to expect. Both pairs, played individually, sounded pretty good to me before!
I may be selling off some Naim gear in the not too distant future to finance this leap into the dark!
G