I recently had my old pair of '57s restored by Kent McCollum, and he added a protection circuit. (I actually don't know what would happen if that kicked in).
I'm using a freshly restored pair of Quad IIs with really good glass. I'm playing all kinds of stuff, early hard rock, post psych proto prog, etc. They aren't terribly efficient, but once they get to a certain loudness, there is so much there--and the bass that is there is taut, tonally rich and well defined in space. I don't need to crank it to stadium sound system levels to "get" the music.
My "big" system, with horns, and SET amps, while very efficient, requires that I give it some power to fully energize the system to its potential, but I'm still not listening at aircraft take off levels. The ability to hear what's going on in the recorded performance is there without playing the system extremely loud.
I think every recording in a given room has a "natural" loudness level where it sounds right without sounding like it is being played at you. This may have a much to do with the room and set up as the equipment itself.
It's been decades since this pair of Quads has been in action-- I did have the Crosby 63, which was a "better" speaker in many respects, but to my ears, was not as coherent in the midrange as the original.
Enjoy them. They are a treat to have, especially if in top condition with appropriate amplification.