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.
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.