(sorry for long post)
Agree with georgehifi that Quad ESL57’s are an icon of speakers (and ESL 63's as well for that matter) and any BS being spread around them needs to be squashed. Why invest in meaningless stuff and even recommend others to do the same on forums like this.
Every ESL63 has a neon lamp that shows that (if in perfect condition) the panels need recharging only roughly once/minute using a fraction of a second current pulse from the HV power supply -- in other words the panels producing your sound are actually connected to mains and your esoteric and expensive power cord during a fraction of a second every minute. It then must be the mere presence of those expensive cables that produce this vast quality improvement you describe:
more natural, clearer, bottom end more dynamic, and so forth, but the lowering of the noise floor, brought forth detail, more natural body, warmth, and a pitch black background, i.e. a grayness or veil in comparison gone, and I mean gone.
I would even say you would get the a bigger quality improvement by buying a bunch of *very* expensive power cords (preferably bi-directional and water-filled) and putting them on the floor infront of your stereo (not connected to anything), since they would at least remove some unwanted floor reflexes.
You can do a quick A/B test by turning off your right channel speaker and connecting a cheap, low quality power cord to your left channel ESL and flip the on/off switch that speaker. In the off stage the ESL will keep its charge for a long time and neither be connected to the lousy power cord nor all the noise coming from your mains outlet.
You will then notice that you can't hear ANY difference att all in sound quality.
Please note I'm not questioning your subjective observations, just our human brains are really lousy in terms of placebo thinking -- if we want to hear something changing, we definitely hear it.
Same thing happened to me when borrowing a really expensive signal cable which my friend said was fantastic. I was extremely sceptical but very surprised when I heard substantial changes --- a
more natural, clearer midrange and treble, some veil gone -- making me sit for a few hours, swapping between my favourite CDs.
It then dawned on me that I was listening to CD, and I had connected the new fantastic cables between my preamp and DAC (not the CD)....
If you want to make improve your ESL57s or ESL63s *for real*, I would suggest to do one or more things of the following in the list below (not necessarily the full list, these are obvious):
(every item possible for DIY or using pros like Kent McCallum or Sheldon Stokes, OTA in Europe)
- refurb your panels if older than, say 15-20 years (YMMV)
-- 57s and 63s
- upgrade your HV power supply --- 57s
- get a proper stand with the optimal height --57s and 63s
- fasten better or even remove some of the obstructing metal screens -- 57s, but only if no children or pets in the house (keep dust covers)
- improve input components (esp. bipolar cap) -- 63s
- replace all crossover caps and resistors -- 57s
- power off your ESLs for a day and then use a pair of good headphones in parallel with one of your speaker connections to check if there is any difference in sound quality (e.g. sibilants on female voices) when connecting/disconnecting the amp to the (quiet) speaker. If it is, upgrade your amp -- 57s and 63s (any speaker really)
- spend LOTS OF TIME trying out the best speaker placement (distance from rear wall, angle and distance to listener, height, and of course room treatment with diffusors and of absorbers.
All of the above items provide REAL improvements, IMHO.
Forget everything about power cord BS, sorry to say.
ESL addict and audiophile for many, many years...