My First Experience of 57's in use was in the 90's probably early 90's when Tim De Paravicini demonstrated Valve Direst Driven Quads at a Audio Event I was attending at, from recollection be music being used was Pink Floyd - Pulse - Master Tape.
Being Blown Away was an understatement.
I also heard them in use by another enthusiast when I was a user of both Single and Stacked array's, (I've never really prejudiced against the Single Array). The enthusiast was a Quad Obsessed Type with experiences acquired for 57's and also had good electronic skills. Panels were Overhauled and the used panels were matched to 0.5dB from recollection. The Stacked Quad enthusiast visited was owning a Pair produced to their design, completely stripped down and rebuilt using only one set of Electronics to Drive the Speaker Design.
Thus single electronics design, was built into a extremely rigid structure, using Black Walnut as the Framing, these really shone as a Speaker. The Source and Amplification was poor for what a Quad ESL 57 really deserves, but this speaker design really shone for its capability.
My Gut Instinct is that the Typical Stacked Array will have been noticeably bettered by the new design for the Stacked Array heard. Especially with a improved Source and Amplification. This is also a design I would really enjoy putting together as a retirement project, where I would be reliant on a EE friend who renovates 57's. Getting the Panels matched to 0.5dB will for me not be something achieved unless having luck with me.
As for the Super Tweeter, I have heard the Super Tweeter from Townsend in use with 57's, but can not recall anything really memorable and requiring to be wanted to be reproduced, but maybe that is because I never heard them coupled as a A/B demo with them cut out and on use, as I have had demo's with quite few Subwoofers I have heard coupled to the Speakers.
As for the High Frequency / Low Frequency Driver Designs to be attached to the Quad ESL 57.
My own intention is to one day have a Audax Gold Dome Piezo HD-3P Tweeter coupled to the 57's. This Driver is one which I and friend have collected samples and devised methods for refurbing the and re-charging the reservoir. This is then enabling the Driver to become very usable for long periods of time, longer that the design from the Manufacturer. My take and my friends take, is that these might be one of the best Upper Frequency Drivers ever produced for the end sonic they are able to deliver. The Driver produces sound by stretching the Gold Coated Diaphragm, which is very much a design to be coupled with a Film that produces an energy to be converted to sound. The Subwoofer choice is a Ripole Design, just to see how a figure of eight radiation will blend with the 57's figure of eight radiation pattern.
My friend who refurb's the ESL 57's has chosen their own was of assembling an array when they take their 57's out to be publicly demonstrated, the Speakers are reorientated to have the Panels Treble and Bass Panels longest sides placed on the Horizontal. The Speakers are usually Spread across an are of approx' 16 feet.
My Amp's had Tranx's specifically wound with the 845 Valve to be used to Drive the Quad ESL 57. When out at shows doing Public Demo's of my Audio System, the most common asked Question is about the Subwoofer I have selected to be used with the Subwoofer. Quite strange facial expressions are seen when informing there is not a Subwoofer. On one occasion, a Show was local to one of the UK's most renowned overhaulers of the ESL 57. A show attendee who new the Company owner personally contacted the owner, encouraging them to attend and listen to my Quads. I was told this during the latter part of the show on Day 2. The Company owner did attend, but did not introduce themselves, so I didn't get the chance to inquire about their own impression of the set up.
Quads with the correct approach can present Bass Just Fine, I learnt that in the room with TDP and the Valve Direct Driven Design. For my purposes, my own design for them produces and end sound and Bass very similar to what TDP had achieved, my recollections used many many years ago reassured myself of this. In the modern day, when I introduce my owned Three Way Floor Standing Cabinet Speakers into the System, the only Bass Issue is the Cabinet Bass and Panel Bass are noticeable for their differences, but not the extension differences that might be different for each Speaker Type.
Recently I have heard a Cabinet Speaker in a friends system, that I believe would easily trounce the ESL 57, I could even buy into the Design as it is a Troels Gravesen design for a Speaker. Interesting one this one, as I recently learnt that Troels has seemingly used Quad ESL's and EAR Amp's designed by TDP to learn about how a Speaker can be Voiced.