Anyone have experience with a Quad 405 amp with Quad 57s?

I have been using modded 405-2 amps with ESL 57s for more then 25 years.  I leave amps and speakers powered 24/7.

The 57 has a rollercoaster of an impedance curve. From 30ohms at 40hz to 1ohm at 20khz. The M3 is voltage-limited at 30ohms and runs out of power. A solution is to use a sub/x-over set around 80hz if you want to play power music like Mahler and Strauss loud.

The Quad 405 is an excellent amp and does not roll off the frequency extremes. I have a nicely restored/modded one to use with my 57’s. It actually is too powerful for the 57’s (27volts/90wpc) so a careful hand on the volume control is necessary to avoid damage. The stock 57’s will arc with more than 20 volts (50 watts). In the past I used a Bedini 25/25 (14 volts) with my first pair of 57’s. No problems. The 405 was designed for the about-to-appear 63. A safer amp for the 57 is the 303 (35wpc). I also have one. Your restored 57’s with the voltage clamp should be OK with the 405.