Quad ESL 57 playing Prince

But beware: max peak voltage is 33 Volt, and the amplifier has to be unconditionally stable, also under capacitive load. These are speakers that have been known to destroy amplifiers, and some amplifiers have been known to destroy these speakers. Impedance is in the 15-30 Ohm range, and falling at higher frequencies. The Quad 303 has the perfect specifications.
The difference with the modern models is not at all night and day, but I do believe the modern 2805 is superior: lower and tighter bass, more power handling, and a spatial performance that is a vanishing act if there ever was one.

No, with a vintage Quad your reservations are "bass-less"!  ;)

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. 

I've had the privilege of listening to more speakers than I remember, and owning a lot of them.  The Quad ESL57 remain the BEST loudspeakers I've come across.

Quads play louder than most people claim.  I'm an 95 dB at the listening chair guy, and I've never seen a problem with the Quads making that volume or encountering any issues related to it.  And while they don't put out that last octave, the bass they do produce sounds awfully good.

That said, if you notice any arcing (looks like sparking), ease up on the throttle, and consider installing the well-known protection circuit

Thanks all. Good to know. I use a pair of rebuilt and updated EICO HF35 vintage monos using 6L6s on the 16ohm tap. I have a REL sub for when it's needed!