Quads ESL 57's don't really have much going on over 15k. 63's similar, the later 989 etc. more.
For 57 correct, although we can have them do much better than that, for 63 you couldn't be more wrong.
So much hearsay talk on these speakers and so much totally wrong info and claims.
The 989 and all later 6-panel Quad ESL's are just a marketing thing, demand for bigger (more impressive) and more bass.
Slam in two extra panels and change nothing is not a good thing. Peter Walker would have never allowed these 6 panel models on the market like this.
Adding an extra 320 pF load to the step-up transformers without any any changes in design is a big nono for me. Just for an extra 5 Hz on the low end.
The 6 panel reviews are all two letters, starts with a B and ends with an S. Commercial blahblah, nothing more, nothing less.
The 4 panel models are what the speaker's electronics and transformers are designed for, starting with the 63. The later models have a much worse metal grill than the original 63 model. Yes the newer models after the 988/989 are a bit more sturdy.
So the 63 model is the only right way to go. How I know? I rebuild these speakers (all models) on a very regular basis with my brother who started a business for it.
Just look at the Quad ESL-63 measurements versus the Maggie (taken from Stereophile review) below.
Tried to get the same dimensions to give a good compare, but can't see what the high limit on the right hand side of the LRS graph is.
If rebuild correct the 63 model can do up to 22 kHz without any problem. I measure them after rebuild about every week.
They have the lowest distortion possible, also plain room measured, of below 0,1% (THD 2-9 that is) above 100 Hz and not rising above 1% in the low, down to 30 Hz
Frequency response, again measured in plain room conditions, plus and minus 6 dB between 30 Hz and 22 kHz with 1/6 octave smoothing.
Evenness L-R within 1 dB over the whole frequency range
I would love to send a set of Quad ESL Plus 63 rebuild by us over to the Schiitr to give people the opportunity to listen to how they can perform with Yggy, Kara and Tyr.
That is the gear I measure them on after every rebuild set that is delivered with a full individual measurement report.
If someone will pay for the transport (NL - USA) I will gladly donate a set.
The LRS
The Quad ESL Plus 63
Evenness L-R Quad ESL Plus 63
Distortion Quad ESL Plus 63