ESL57 vs ESL63

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.

@bdp24, interesting I didn't know that about the 57s. I will look into the Rhythmik subs.

I owned the 63s and quite liked them. But a few friends had the 57s and I actually liked the tone of the 57s more - a bit more present and warm sounding to my ear, and a bit more solid sounding. I’ve wanted to own the 57s for a long time, if only as a pair of "use them sometimes" speakers. But their form factor simply doesn’t work anywhere in my house.   I was actually offered a beautiful pair for FREE and couldn't take them!