I preferred the 57 to 63 in spite of its very narrow sweet spot, limited power handling and awkward size. I found the 57 made music appear much more realistic. By contrast the 63, whilst still a magnificent speaker, sounded relatively recessed; a case of “technically” better but the illusion of having musicians in front of me wasn’t so convincing. Listen to them without grilles, cloth and dust sheets (not recommended for safety and speaker life) and the sound comes alive. The later versions with the rear brace (2805 on) seem to have addressed the 63s sound quality issues and sound much more alive.
Even with the 63s shortcomings, to my ears, they still sound much more like listening to live music then most box speakers including the latest finest measuring speakers. For a single listener, particularly for classical, I found the 57 the most convincing. For anyone who puts measured response ahead of realistic sound then the 63s are arguably better.
The biggest problem with electrostatics is that once heard, depending a bit on genre of music, few other speakers will be acceptable. They also pull off that trick of reproducing every detail in the music without becoming fatiguing in the way that a monitor speaker can. Listening sessions always end up going on longer than intended.
In spite of my reservations about the 63 both Quad models are excellent speakers but are now long in the tooth and servicing costs need to be taken into account. The latest models certainly sound superb but again service life has to be taken into account. In the past year or so I compared them with speakers costing up to £30k and to my ears they bettered them all by a considerable margin.