I sure miss my Quad ‘57s!

I can’t say whether the rebuilders get more bass and treble from the speaker, but there have been improvements to the power supply and the Zener clamp boards allow you to use a wider selection of amps without the fear of arcing the panels. As previously mentioned Electrostatic Solutions is a good resource to learn about some of the design improvements.

As for the RM10 and the ESL 57 well it was somewhat by accident that the amp was designed with that speaker in mind. It just so happened that was the speaker Roger was using at the time so he certainly wanted an amp that could play well with it (actually right to the very end Roger had his ESL 57 running in the shop for our music pleasure). The RM10 is very balanced with the ESL 57, but most amps will have trouble keeping up with the rise in the bass frequencies and the RM-10 could be perceived to be a bit lean there, but overall it’s one of the best tube amps to use with that speaker.

OTL amps can deal with that rise in the bass, and along with the RM10 I have used the Atma-Sphere M60 with these speakers and even though their output impedance is much higher than the RM10, it plays quite well.

I find tube amps that only have 8ohm taps usually sound a bit soft and rolled off in the highs with the 57's, as they are a 2.5ohm load already at 10khz and falling to 1ohm at 20khz.
This is why I prefer a good stable 30-40w Class-A solid state that can do the doubling, I built quite a few of these for 57 owners based on the Nelson Pass A40 and they loved them.
http://www.firstwatt.com/pdf/art_a40.pdf

This is why they say they best amp ever for the 57's, was the Mark Levinson ML2 25w Class-A monoblocks, that could double down to 1ohm.
 https://img.ukaudiomart.com/uploads/large/2229206-mark-levinson-ml2.jpg

Cheers George    

I gave up on my Quad ESL 63s in the late 90's and don't miss them.
I'm definitely a dynamic speaker guy but I WOULD like to own a pair of ESL 57s as well.  I like them better than the 63s (though, ultimately, not as much as some dynamic speakers).

I bought my first pair of 57's back in '81. Along with a pair of Futterman H3aa OTL amps. And around that time a Bedini 25/25 class A amp. I alternated between those two amps. In 2001 I stupidly sold the Quads and the Futtermans. This I soon regretted! Now thanks to good fortune in the past few years I've acquired two pairs of 57's and three Futterman amps!

The 57's in stock form are limited to 20 volts input (50 watts). More than that they will arc! Stay below that and they will be fine! Do not expect to get floor-shaking bass and window-rattling spl's out of them! They are not JBL's!