I think ESL 63 has similar trait to 57, correct?
Yes and mainly NO!
Both are electrostatic, so have no box around them to resonate. The almost-as-light-as-air diaphragms have no overshoot unlike massive conventional drivers. Both are meant to be full range speakers.
But the original ESL (now known as the 57 because 1957 was when it was released) was not conceived for stereo. It dealt with the problem of flat panels by curving them slightly about a horizontal axis. It is a three-way design with three panels per speaker and crossovers and compared with modern speakers lies on its side.
The ESL 63 was conceived in 1963, after stereo records became commercially available, but the 63 was not released until 1981. One of the objectives was to reduce the treble beaminess of the ESL 57 and another was to ensure a long service life - not easy with ultra-light diaphragms charged to over 5,000-Volts in a narrow gap between stators which carry the audio signal transformed to high voltages. Peter Walker named the 63 "FRED" for full-range electrostatic dipole. With minor modifications, it and its successors are still in production.
Note that these have no conventional crossovers - another source of distortion removed. Overly high stator voltages are eliminated by attenuating signal voltages above 40-Volts. To reduce the possibility of sparking which can burn holes in the diaphragm, an ionization detector could shut the speaker down by in effect making it look like a short circuit to the amplifier.
Alastair Robertson-Aikman, the founder of SME, tried various modification to the ESL-63 including stacking them. Quad themselves introduced models with 50% more panel area to extend the low bass.
In the last few weeks, Quad has introduced two models with an -X suffix. One of the things they seem to have done is to add insulation to the stators, and to remove the ionization detector in order to get a bit more volume.
My only real criticism of these speakers is that they don’t play loudly enough in big rooms, especially with the dynamics available from high resolution sources. One way to boost their apparent output is to pair them with subwoofers.
Although they are big panels, the concentric stator design means they can also be used as near-field monitors which is an easy way to effectively crank-up the apparent volume.
A second criticism is the life of the panels, especially when they were produced in China. The story is that Peter Walker specified glue with a 10 year minimum service life and the glue maker took him too literally
