Jadis JA30 - Quad ESL63 .. 4,8 or 16ohms?

Sorry to disagree, but the ESL63 has an impedance that increases with sound level in the bass. At levels over 100mV the impedance never drops bellow 8 ohm in the bass. People usually measure impedance with a 1000 ohm series resistor, using levels between 10 and 50 mV, and assume that impedance does not depend on level. This is not true for the ESL63 and was published in the original HIFI NEWS review in 1981.
I have owned (and still keep) ESL63 and always used them with the 8 ohm tap of jadis, conrad jonhson and audio research tube amplifiers.

4est, you are discussinging the Quad ESL63 with the same ideas that are usually applied to electrostatics such as SoundLab and Martin Logan, but the ESL 63 is a different case. SoundLab and MartinLogan have impedances that can be as low as 1 ohm, but the ESL63 never goes lower than 4 ohm, being 5 ohm at 10kHz. Unless you listen to a lot of heavy rock and electronic music the power of music at frequencies above 5 kHz is much lower than average power - I have measured it with a spectrometer. Some versions of Shostakovich symphonies or some challenging test disks have peaks that are exceptionally rich in high frequencies, but most of the music some one typically listens in ELS63 is not a problem. Remember Peter Walker designed the ESL63 for the Quad 405 that was also current limited.
May be you prefer the 4 ohm tap because with this tap the damping factor of the amplifier is more adequate for your taste and listening environment.
I did some research and plenty of measurements on electrostatics because I own an amplifier that sounds lovely with electrostatics but it is current limited - the Dartzeel.
If someone has interest in the original HifiNews 1981 measurements I will be pleased to email them.