ESL57 vs ESL63

Since my subs are located where they provide the flattest response, sitting a 63 or a 57 On top would work well enough, i’d have to figure out how to use my subs for both systems, i use a MiniDSP. They are quite capable and musical, they are sealed Adire Tumults from the 2005 era, i use (4) for my HT.... my guess one 15 would suffice.

I dont think a blind purchase of a pair of 63’s would be reckless, an electronic XO set to 80ish hz would remove a lot of motion from the panels, a nice XO...bryston or a tube Marchand, they are here in Rochester 

Gradient made dipole subs for the ESL 63. Here is an article from Robert E. Greene discussing the set up:

http://www.regonaudio.com/Gradient%20SW-63%20Subwoofer.html

The ’57’s are some of the most coherent speakers I have ever heard in the mids to the lower treble. The ’63’s are not quite as coherent, but they are a little more extended in both frequency extremes, IMO. Stacked ’57’s are pretty amazing IF you can accommodate their requirements.
Now, here’s the thing, IMO and IME. Neither is a speaker that I would want for my primary system, too many limitations and restrictions with both. Many times, i contemplate a great pair of ’57’s in a secondary system...BUT as a secondary system only.

BTW, IF you go for either...a nice mid powered tube amp is the way to go...like an ARC D70Mk2 that I used to own. Match made in heaven.

@clio09, Gradient also made an OB/Dipole sub for the 57. It looked a lot like the current Rythmik/GR Research, but laid on it’s side. Danny Richie says the R/GR can be so positioned as well, and used as a base for the 57. I would install a set of Townshend Audio Seismic Pods between them, to prevent vibration from being transferred from the H-frame into the 57.

Wow, such diverging opinions. The 57s do not come into their own until they are stacked but without subwoofers they remain fragile and even with subwoofers they are not hard to blow. I was present for the destruction of two 57s in an HQD system. Peter would get a little carried away showing off. Only Decca ribbons blew more frequently. 
cd318, "a sideline"? You obviously have not listened to big Acoustats or Soundlabs. For $40,000 you can get an indestructible loudspeaker that to many ears sounds better than any other loudspeaker regardless of price.
I know my opinion does not count much because I am so pretentious but just ask Audiokinesis or Atmasphere. A sideline? I hardly think so.